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Abstract

Many people worldwide want to be less neurotic. As lower levels of neuroticism predict a variety of positive life outcomes, including health, success, and relationship quality, interventions that help people to become less neurotic could have wide-ranging impacts. This study protocol describes the rationale and design of the CHILL (Changing How I Live Life) Study, which aims to test and evaluate a smartphone-based intervention to reduce neuroticism in the general public. The CHILL Study comprises four survey assessments spread over 5 months, a 6-week intervention phase, and two 1-week experience sampling phases without interventions. During the intervention phase, participants motivated to become less neurotic are randomized to four core treatment groups. These treatment groups differ in their employed intervention strategies that target different aspects and levels of neuroticism. The CHILL Study moves beyond existing volitional personality change research by testing which strategies most effectively decrease neuroticism and evaluating the longevity and generalizability of intervention-induced neuroticism change via peer-reports, implicit personality assessments, and a 7-week follow-up period. The goals of the CHILL Study are to generate an effective method for decreasing neuroticism and facilitating the development of a comprehensive theory of neuroticism change.

Keywords

personality intervention volitional personality change neuroticism smartphone multimethod

Neuroticism describes the tendency to frequently experience negative affect, show strong emotional reactions to stress, and perceive the world as a threatening place while doubting one’s ability to deal with such threats (Barlow et al., 2014; Goldberg, 1990; Soto & John, 2017). Neuroticism predicts a range of detrimental life outcomes (Beck & Jackson, 2022; Soto, 2019; Wright & Jackson, 2023). It has been associated with the onset of mental disorders and physical health problems, the occurrence of major life stressors such as divorce or job loss, and increased risks for long-term disability and mortality (Barlow et al., 2014; Goldstein et al., 2020; Jokela et al., 2013; Lahey, 2009; Roberts et al., 2007). Altogether, the costs associated with neuroticism exceed those of common mental disorders (Cuijpers et al., 2010). Measures that can effectively reduce neuroticism may thus offer promising ways to promote health and well-being in the general public.

With evidence that many people want to become less neurotic and are willing to invest time and money to do so (Baranski et al., 2021; Hennecke et al., 2014; Miller et al., 2019), the question of whether trait levels of neuroticism can be changed volitionally has recently gained increased attention (Bleidorn et al., 2019; Hudson & Fraley, 2015; Stieger et al., 2021). Volitional personality trait change describes self-directed personality changes in desired directions, for example, through targeted interventions (Hudson, 2022; Hudson et al., 2019; Stieger et al., 2018). Although there is initial evidence that such interventions can successfully promote desired personality changes (Haehner, Wright, et al., 2024; Roberts et al., 2017; Stieger et al., 2021), it is still unclear (1) which intervention strategies are successful in changing neuroticism, (2) whether intervention-induced neuroticism changes generalize beyond self-reports, (3) when interventions should be delivered to be most efficient, and (4) which contextual or personal characteristics moderate intervention efficacy. A thorough investigation of volitional neuroticism change is thus needed to fully realize its potential. This study protocol describes the rationale and design of the Changing How I Live Life (CHILL) Study, a smartphone-based intervention designed to reduce neuroticism in the general public.

Neuroticism as an intervention target

Neuroticism is a risk factor for many mental disorders – particularly those related to depression and anxiety – and accounts for much of the genetic vulnerability underlying these disorders (Clark & Watson, 1991; Griffith et al., 2010; Khan et al., 2005; Weinstock & Whisman, 2006). Public health experts have thus called to shift from interventions that focus on specific types of mental disorders to interventions that focus on neuroticism as the root of these problems (Barlow et al., 2014; Cuijpers et al., 2010; Widiger & Oltmanns, 2017). This suggestion is supported by recent evidence suggesting that both the levels of neuroticism and changes in neuroticism can predict important life outcomes. For example, decreases in neuroticism have been found to predict a better health status and reduced mortality risk several years later (Human et al., 2013; Mroczek & Spiro, 2007; Wright & Jackson, 2023). Together, these findings make neuroticism a prime candidate for interventions designed to improve public health and well-being (Bleidorn et al., 2019).

However, changing a personality trait like neuroticism is challenging. Contemporary theories of personality development attempt to explain why personality traits remain relatively stable over time and which conditions may lead to lasting personality trait change (Bleidorn, 2024). For example, the TESSERA Framework (Wrzus & Roberts, 2017) suggests that personality traits only change if people experience lasting shifts in the dynamic interplay of experienced situations, expectations, personality state expressions, and reactions. Thus, this model predicts that personality traits do not change if one of these elements does not change or if shifts in these dynamics are not lasting. Similarly, Whole Trait Theory (Fleeson & Jayawickreme, 2015) posits that lasting personality trait changes only occur if the explanatory mechanisms that generate personality state expressions in daily life are changed. Mechanisms like interpretation of situations, homeostatic forces, or activation of goals typically result in relatively stable distributions of personality states. Finally, the Theory of Self-Regulated Personality Change (Hennecke et al., 2014) outlines three preconditions of successful personality change. The first precondition is that people want to change their personality traits or trait-related behavior, such as being more relaxed in stressful situations. The second precondition is that people consider personality change feasible and believe that they are able to change their personality in the desired way. Then, the third precondition is that people have the capacity to show enduring changes in trait-relevant behaviors (e.g., react more relaxed in stressful situations, show efficient coping strategies) so that these behavioral changes become habitual and extend across situations and life domains.

In sum, these theoretical perspectives may explain why neuroticism remains relatively stable within individuals and the population (Bleidorn et al., 2022), despite many people wishing to become less neurotic (Baranski et al., 2021; Hudson & Fraley, 2016). Merely desiring to become less neurotic thus does not seem sufficient to promote lasting changes; rather, effective interventions may be needed to change neuroticism in the general public.

Interventions to decreases in neuroticism in the general public

To date, the best evidence that neuroticism can be changed through intervention comes from clinical studies. In a meta-analysis, Roberts et al. (2017) found that psychotherapeutic interventions can lead to lasting decreases in neuroticism (−0.57 ≤ d ≤ −0.39), illustrating that intervention-induced neuroticism change is possible in clinical populations. However, this line of research provided no evidence on how neuroticism can be changed and whether change is possible with less resource intensive interventions that can be offered to the general public.

Initial insights into these questions have been provided by volitional personality change research that examined how personality traits can be changed in nonclinical populations (Haehner, Wright et al., 2024; Hudson, 2021b). In one of the key studies on this topic (Stieger et al., 2021), participants chose how they wanted to change their personality, with one option being to decrease in neuroticism. Over 10 weeks, participants then used a smartphone app with more than 10 different intervention strategies such as resource identification, goal setting, and motivated reflection (Allemand & Flückiger, 2017). The authors found that the smartphone intervention led to a lasting decrease in participants’ neuroticism levels (d = −0.54) that also generalized to other outcomes such as well-being (Allemand et al., 2024; Olaru et al., 2023; Stieger et al., 2023). Similarly promising results were also found in a study with a personalized coaching intervention (Allan et al., 2018; Martin et al., 2014) and in a study that examined how completing behavioral challenges fosters neuroticism change (Hudson et al., 2019).

However, this initial evidence requires replication and extension as several open questions about intervention-induced neuroticism change remain unanswered: (1) What are the key components of effective neuroticism interventions? (2) Why do people differ in how they respond to interventions? (3) How can the efficacy of neuroticism interventions be optimized for the general public? (4) Under which conditions are neuroticism interventions most effective? To address these questions, novel empirical approaches are needed that move beyond one-size-fits-all approaches and that allow researchers to identify which intervention strategies are most successful in reducing neuroticism (Wright et al., 2024).

An empirically derived taxonomy of intervention strategies to reduce neuroticism

There currently exists little consensus about the most effective strategies to reduce neuroticism – possibly because a common terminology for describing and classifying neuroticism interventions is lacking (Wright et al., 2024). For example, the most common description of psychotherapeutic interventions to reduce neuroticism in the meta-analysis by Roberts et al. (2017) was “mixed” – providing little information on specific strategies to successfully change neuroticism. Similarly, existing research on volitional personality change differs significantly in its approaches (Haehner, Wright et al., 2024), precluding a systematic investigation and knowledge accumulation of the efficacy of different intervention strategies.

To overcome these limitations, Wright et al. (2024) recently proposed an empirically derived taxonomy of neuroticism interventions for the general public. In line with theoretical conceptualizations of personality traits (Allemand & Flückiger, 2017; Geukes et al., 2018; Hennecke et al., 2014; Jackson & Wright, 2024; Wilt & Revelle, 2015; Wrzus & Roberts, 2017), this taxonomy distinguishes relevant intervention strategies based on the levels and aspects of intervention targets. The level of intervention targets describes whether a strategy intervenes at the state, habit, or trait level of neuroticism. State-level interventions (e.g., relaxation) address momentary manifestations of neuroticism such as instances of stress, negative affect, or worrying. Habit-level interventions (e.g., cognitive restructuring) target domain-specific pairings of thoughts, feelings and behaviors, whereas trait-level interventions directly address stable, dispositional patterns of thoughts, feelings, and behaviors that generalize across situations (Allemand & Flückiger, 2017; Fleeson & Jayawickreme, 2015; Wrzus & Roberts, 2017). The aspect of intervention targets refers to whether an intervention strategy focuses on affective, behavioral, and/or cognitive aspects of personality (Wilt & Revelle, 2015). As personality traits like neuroticism are multifaceted constructs, each level of neuroticism is comprised of representative affective aspects (e.g., negative affect), behaviors (e.g., isolating from others), and cognitive aspects (e.g., worrying). Thus, intervention strategies can focus on different aspects of neuroticism.

Based on a systematic review of neuroticism interventions in nonclinical populations, Wright and colleagues then identified 19 distinct intervention strategies that differ in their level and aspect of intervention targets. Table 1 provides an overview of this empirically derived taxonomy of neuroticism intervention strategies. For example, Psychoeducation is a general intervention strategy that gives people relevant information on how to change their neuroticism, primarily targeting cognitive aspects of neuroticism at the trait level. Motivated Reflection – prompting people to reflect on their thoughts, feelings, behaviors and identity – is an intervention strategy that targets cognitive aspects of neuroticism at the habit level.

Table 1.

Overview of neuroticism intervention strategies.

Strategy	Definition	Level	Aspect
Behavioral activation	Increasing engagement in valued life activities to elicit improvements in thoughts, mood, and quality of life	State	B
Cognitive restructuring	Guiding the identification and modification of unhelpful or negative thoughts or beliefs	Habit	C
Discrepancy awareness	Making explicit the discrepancy between one’s actual and ideal self	State	C
Emotional support	Prompting to claim assistance or comfort from close others	State	A, B, C
Exposure to humor	Presenting stimuli to elicit positive emotions	State	A, C
Exposure training	Confronting with feared stimuli to prevent avoidance, elicit habituation, and increase self-efficacy in a safe environment	State	B, A
Goal setting	Prompting the creation of specific, time-based behavior targets that are measurable, achievable, and realistic	Habit	C
Implicit bias modification	Presenting a cognitive task to implicitly focus more on positive instead of negative cues	Habit	C
Information feedback	Presenting information on task performance or intervention progress	Trait	C
Mindfulness and meditation training	Fostering awareness of internal states by learning to observe momentary feelings, thoughts, or experiences without judging or reacting to them	State	C, A, B
Motivated reflection	Encouraging to reflect on their emotions, thoughts, behaviors, and/or identity	Habit	C
Physical activity	Prompting bodily movement that increases energy expenditure and is meant to elicit a beneficial physiological response	State	B
Positive reinforcement	Rewarding an adaptive behavior to increase the probability of it happening again in the future	Habit	B
Psychoeducation	Providing information that is intended to change their trait neuroticism	Trait	C
Relaxation	Providing techniques that produce calmness of mind, body, or both, or otherwise reduce pain, anxiety, stress, or anger	State	B, A
Resource identification	Prompting the identification of personal, social, or professional means or entities that can provide aid or support in times of need	Habit	C
Skill building via modeling	Guiding the acquisition training of skills through watching others perform, either directly or via media such as films and videos	Habit	C, B
Skill building via practice	Guiding the acquisition training of skills through performing or practicing it yourself	Habit	B
Structural or practical support	Providing access to practical or structural resources to aid in alleviating burdens or problems	Habit	C, B

Note. Table adapted from Wright et al. (2024). The intervention strategies Exposure and Implicit Bias Modification are not implemented in the CHILL Study because they could not be readily implemented in a digital smartphone app. More details on the different intervention strategies and their relation to contemporary theories of personality change can be found in Wright et al. (2024).

Overall, this taxonomy of neuroticism interventions provides a systematic overview of existing intervention approaches and offers a terminology for describing and classifying relevant intervention strategies. Applying this taxonomy to practice may thus help to identify which ways of targeting neuroticism are most successful. Therefore, the implementation of intervention strategies in the CHILL Study was based on this taxonomy to allow for a closer examination of successful intervention approaches to change neuroticism. However, the efficacy of intervention strategies may vary within and between participants (Hudson, 2021b; Klasnja et al., 2015). To be able to examine such effects, another critical feature of the CHILL Study was the implementation of intervention strategies by means of a smartphone app.

Using digital interventions to promote decreases in neuroticism

Existing research on intervention-induced neuroticism change shows that effects differ across studies and participants (Haehner et al., 2024; Hudson, 2021a; Hudson et al., 2019; Stieger et al., 2021). Understanding when and why the effects of neuroticism interventions differ across people may be critical for maximizing their efficacy. One way to test these questions is using digital interventions that allow researchers to move beyond one-size-fits-all approaches and provide scalable solutions to individually tailor interventions.

Three features move digital interventions beyond traditional treatment approaches. First, the highly standardized nature of digital interventions allows for the manipulation of individual intervention aspects while controlling for others, providing researchers with an experimental paradigm to identify relevant characteristics of successful neuroticism change (Klasnja et al., 2015). Second, advances in internet and smartphone technology have made it possible to capture intensive longitudinal data on psychological change processes with limited expense for researchers or burden for participants (Moshe et al., 2021). This allows researchers to obtain large sample sizes to conduct sufficiently powered trials and ultimately make interventions available to the general population (Stieger et al., 2021). Third, the ability to adapt the timing and amount of interaction with specific intervention strategies enables the development of personalized intervention approaches, potentially boosting their efficacy (Montag et al., 2020; Nahum-Shani et al., 2015).

The CHILL Study

This protocol describes the rationale and design of the CHILL Study, which aims to test the efficacy of a digital intervention to change neuroticism in the general public. Our intervention is guided by the taxonomy of intervention strategies by Wright et al. (2024). Based on this taxonomy, we plan to create four core treatment groups (TGs) that receive different intervention strategies. Specifically, TG 1 receives trait-level interventions (e.g., psychoeducation); TG 2 receives trait- and habit-level interventions (e.g., psychoeducation and goal setting), TG 3 receives trait-level and state-level interventions (e.g., psychoeducation and physical activity), and TG 4 receives trait-, habit-, and state-level intervention (e.g., psychoeducation, goal setting, and physical activity). With the CHILL Study, we aim to address three broad research questions: Does the intervention lead to a decrease in neuroticism across TGs? Does the efficacy of intervention strategies vary across TGs? Do the effects of the intervention generalize to other life outcomes and peer-reports of neuroticism?

Methods

Transparency and openness

Data collection for the CHILL Study starts in June 2024 and is expected to be completed in December 2024. The design of the CHILL Study was preregistered at https://osf.io/mc7v3/?view_only=27cf0b64a71d4445945f6885818c8069. Data collection of the CHILL Study has been approved by local ethics committee of University of Zurich [Number 23.08.20]. After completing the data collection, we will publish an anonymized dataset on OSF. Hypotheses and specific analysis plans will be preregistered separately for subprojects that will rely on data from the CHILL Study.

Design and procedure

The design of the CHILL Study is illustrated in Figure 1. Participants will first complete a screening that includes information about the study and questions about five inclusion criteria. Specifically, participants are required to (1) speak German, (2) be between 18 and 40 years old, (3) have a smartphone with internet access, (4) be willing to download the intervention app, and (5) desire to become less neurotic. Only people fulfilling these inclusion criteria are invited to take part in the study. Furthermore, participants are asked to invite up to three peers to support their participation in the study. To do so, participants receive unique identification numbers and registration links to send to their peers.

Figure 1.

Overview of the CHILL Study design. Participants first complete a screening to determine their eligibility to take part in the study. For eligible participants, the CHILL Study comprises four survey assessments (T0 to T3), a pre-intervention experience sampling phase, an intervention phase, and a post-intervention experience sampling phase. T0 to T3 survey assessments are conducted in formR, while all other assessments are implemented in m-Path. The intensive data collections in m-Path comprise weekly surveys, daily-diary questionnaires every evening, and experience sampling surveys three times per day. For the intervention phase, participants are randomized to four core treatment groups (TGs) that receive different intervention strategies. Furthermore, participants invite up to three peers to provide peer-reports of participants’ personality traits in four survey assessments. W = weekly assessment; D = daily assessment; ESM = experience sampling assessment.

Procedure for target participants

The CHILL Study comprises four survey assessments conducted in formR (Arslan et al., 2020) and an intervention phase implemented in m-Path (Mestdagh et al., 2023). Participants are invited to the trait assessments (T0 to T3) via email and receive up to two reminders to participate in each trait assessment. At T0, participants provide informed consent and rate their neuroticism using the BFI-2 (Danner et al., 2020). After completing T0, there is a waiting period of 2–7 weeks, depending on participants’ registration dates. During this period, tutorials on how to download and set up the intervention app as well as virtual information sessions with the research team are provided to participants. The waiting period is included to compare the extent of normatively occurring changes with intervention-induced changes. After the waiting period, participants are invited to T1, which has assessments of various trait characteristics, including personality traits, self-esteem, life satisfaction, emotion regulation strategies, affect, personality change goals, and demographic information (see Measures section for details).

After T1, participants receive an individualized invitation to the smartphone intervention app m-Path. On the Monday after registering in m-Path, participants complete a 1-week baseline phase of experience sampling method (ESM) surveys. On each day, participants complete three ESM assessments and one end-of-day diary assessment, consisting of questions about their personality, environment, and experiences of stress and negative affect. Furthermore, for the next 8 weeks, participants receive a weekly questionnaire that, among other things, assesses change goals and perceived personality changes.

After the baseline ESM period, the 6-week intervention phase of the CHILL Study begins. Participants are randomized to four core TGs that differ in their intervention content (see below for details). During this intervention phase, participants receive regular intervention prompts whose frequency and content depend on their TG (see Table 2). Upon completion of the 6-week intervention period, all participants complete another week-long monitoring ESM phase, which again comprises three ESM assessments and one end-of-day diary assessment per day.

Table 2.

Overview of treatment groups.

Intervention strategy	Primary intervention target	Frequency	TG 1	TG 2a	TG 2b	TG 3	TG 4a	TG 4b
Information feedback	Trait	Continuously presented	✓	✓	✓	✓	✓	✓
Psychoeducation	Trait	Continuously presented	✓	✓	✓	✓	✓	✓
Structural or practical supporta	Habit	Continuously presented	✓	✓	✓	✓	✓	✓
Cognitive restructuring	Habit, cognitive	2x per week		✓			✓	✓
Goal setting	Habit, cognitive	2x per week		✓			✓	✓
Motivated reflection	Habit, cognitive	2x per week		✓			✓	✓
Resource identification	Habit, cognitive	2x per week		✓			✓	✓
Positive reinforcement	Habit, behavioral	Task contingent			✓		✓	✓
Skill building via modeling	Habit, behavioral	2x per week			✓		✓	✓
Skill building via practice	Habit, behavioral	2x per week			✓		✓	✓
Behavioral activation	State	3x per week				✓		✓
Discrepancy awareness	State	3x per week				✓	✓	✓
Emotional support	State	3x per week				✓	✓	✓
Exposure to humor	State	Included in videos				✓	✓	✓
Mindfulness and meditation training	State	3x per week				✓	✓
Physical activity	State	3x per week				✓		✓
Relaxation	State	3x per week				✓	✓

Note. This table summarizes the allocation and dosage of intervention strategies across treatment groups (TG). Trait-level interventions are continuously presented in the CHILL Area. TG 2 is split into TG 2a and TG 2b to distinguish cognitive and behavioral intervention strategies. TG 4 is split into TG 4a and TG 4b to distinguish between state-level intervention strategies that require participants to perform purely behavioral activities (e.g., Behavioral Activation) and strategies that also target cognitive and/or affective aspects (e.g., Mindfulness and Meditation Training).

^aAlthough this strategy primarily targets neuroticism at the habit level, it is accessible continuously for all TGs to fulfill ethical requirements.

Then, 8 weeks after the T1 assessment, all participants are invited to the post-intervention survey assessment (T2) that contains the same measures as the pre-intervention assessments. Another 7 weeks later – approximately 15 weeks after T1 – participants receive an invitation for the fourth and final survey assessment (T3), which serves as a follow-up to examine the longevity of the intervention effects.

Procedure for peers

Peers who are nominated by the target participants enter the study via links provided to the target participants. As a first step, peers complete a brief registration which includes information on the study design and determination of eligibility: (1) German-speaking, (2) age of at least 18 years, and (3) not a target participant in the CHILL Study. Eligible peers are invited to the four survey assessments of the CHILL Study that take place in parallel to the trait assessments of the targets. These peer assessments include questions about their relationship to the target as well as peer-reports of targets’ personality traits and perceived personality changes.

Participants and recruitment

We intend to recruit German-speaking adults between the ages of 18 and 40 who are willing to take part in a digital intervention study and who are motivated to decrease their neuroticism. We selected the age range of 18–40 years because personality traits are generally believed to be more malleable in early adulthood than they are during other developmental periods like middle adulthood (Bleidorn et al., 2022; Schwaba & Bleidorn, 2018). Furthermore, in line with theoretical and ethical considerations on volitional personality change, we restrict our sample to people who are motivated to decrease their neuroticism (Hennecke et al., 2014).

Participants will be recruited using convenience sampling methods, including social media platforms, newspapers, mailing lists, flyers, and websites. Participants can receive up to CHF 200 for participation in the study, with compensation being contingent upon completion of different questionnaires across the study period. Participants will also receive a personalized personality feedback report after the study is completed. Finally, peers will be entered into a raffle for 10 vouchers for Galaxus or Amazon worth CHF 50 each.

We aim for a final sample size of N = 390 participants. With this sample size, we will have 80% power at α = .05 to detect an effect of TG assignment on changes in neuroticism across the study period equal to d = 0.44 (f = .222, η² = .047) in line with effect size estimates in existing research on volitional change in neuroticism (Haehner, Wright et al., 2024; Roberts et al., 2017; Stieger et al., 2021).

Measures

Survey assessments and peer-reports (T0 to T3)

Tables 3 and 4 provide an overview of the constructs that are included in the survey assessments (T0 to T3) of the CHILL Study, which comprise questionnaires on personality traits, well-being, and cognitive abilities. T0 differs from the other trait assessments as it only includes an assessment of neuroticism.

Table 3.

Overview of constructs assessed at the trait assessments T0 to T3 (self-report).

Construct	Questionnaire	Items	Example item	Remark
Demographics	—	21	“How old are you?”	Only assessed at T1; response scale differs across variables
Big five traits	BFI-2 (Danner et al., 2020; Soto & John, 2017)	60	“I am someone who is outgoing, sociable”	At T0 only neuroticism items are used
	Big five triplets (Wetzel & Frick, 2020)	20	“I get stressed out easily.”	Participants selects the statement that describes them best
			“I have little to say.”
			“I distrust people.”
	Big five IAT (Back et al., 2009; Carpenter et al., 2019)	60	—	See Back et al. (2009) and Schmuckle et al. (2008) for details
Change goals	BF-TGI (Robinson et al., 2015)	5	“Extraversion: An extraverted person is characterized by being active, assertive, energetic, enthusiastic, open-minded and talkative.”	Items assessed on a 3-point response scale ranging from I have a goal to become more like this trait to I have a goal to become less like this trait
Neuroticism behaviors	—	2	“Being unable to control my mood from changing so quickly”	Participants select the three neuroticism-related behaviors that bother them the most
Self-esteem	RSES (Collani & Herzberg, 2003; Rosenberg, 1965)	10	“All in all, I am satisfied with myself”
Self-control	B-SCS (Bertrams & Dickhäuser, 2009; Tangney et al., 2004)	13	“I am good at resisting temptation”
Self-efficacy	GSE-3 (Beierlein et al., 2012; Schwarzer & Jerusalem, 2012)	3	“I can rely on my own abilities in difficult situations”
Emotion regulation	REQ (Phillips & Power, 2007)	9	“I dwell on my thoughts and feelings”	Only subscales internal functioning and external functioning assessed
Socio-emotional skills	BESSI-4 (Lechner et al., 2022; Sewell et al., 2024)	4	“Find reasons to feel good about myself”	Only subscale emotional resilience skills assessed
Personality functioning	LFPS (Spitzer et al., 2021; Weekers et al., 2019)	12	“I often do not know who I really am”
Maladaptive personality traits	PID-5 (Bach et al., 2020)	6	“I worry about almost everything”	Only subscale negative affectivity assessed
Growth mindset	—	2	“I think it is possible to change my personality”
Affect	I-PANAS-SF (Janke & Glöckner-Rist, 2012; Thompson, 2007)	10	“Active”
Life satisfaction	SWLS (Diener et al., 1985; Glaesmer et al., 2011)	5	“I am satisfied with my life”
Perceived stress	PSS-4 (Cohen et al., 1983; Klein et al., 2016)	4	“How often in the last month have you felt that you were unable to control the important things in your life?”
Depression and anxiety	PROMIS (Pilkonis et al., 2011)	16	“I feel fearful”
Cognitive abilities	ICAR (Condon & Revelle, 2014)	16	“What number is one fifth of one fourth of one ninth of 900?”	Only assessed at T1 and T3; response scale differs across items

Note. Unless not otherwise indicated, constructs are assessed on a 5-point scale.

Table 4.

Overview of constructs assessed at the trait assessments T0 to T3 (peer-report).

Construct	Questionnaire	Items	Example item	Remark
Demographics	—	21	“How old are you?”	Only assessed at T1
Relationship to target	—	3	“How many hours to you spend in a usual week with [target name]?”	Only assessed at T1
Big five traits	BFI-2 (Danner et al., 2020; Soto & John, 2017)	60	“[Target name] is someone who is outgoing, sociable”	Assessed on a 5-point response scale
	Big five triplets (Wetzel & Frick, 2020)	20	“[Target name] gets stressed out easily.”	Peers select the statement that best describes the target
			“[Target name] has little to say.”
			“[Target name] distrusts people.”
Perceived change	—	5	“Extraversion: An extraverted person is characterized by being active, assertive, energetic, enthusiastic, open-minded and talkative.”	Assessed on a 3-point response scale ranging from [target name] has become more like this trait to [target name] has become less like this trait

ESM assessments, daily-diary assessments, and weekly assessments

Table 5 provides an overview of the constructs that are assessed in the ESM, daily diary, and weekly assessments. Invitations for weekly assessments are sent to participants on each Saturday at 10 a.m. (available until 11:59 p.m. on Sunday), daily assessments are sent each day at 8 p.m. (available until 11:59 p.m.), and ESM assessments are sent three times a day at random within two-hour periods of 9–11 a.m., 1 p.m.–3 p.m., and 5 p.m.–7 p.m. (available for 135 min). If participants do not respond to the first notification, reminders for these assessments are sent 75 min after the first notification.

Table 5.

Overview of constructs assessed at the ESM, daily diary, and weekly assessments.

Construct	Questionnaire	Items	Example item	Remark
ESM assessment
Affect	—	1	“How do you feel right now?”	Slider from 0 to 100
Neuroticism state	Items based on Bleidorn (2009)	2	“Please rate the extent to which these adjectives describe you right now”	Bipolar scale ranging, for example, from strained to relaxed
Stressful event	—	1	“Since the last assessment, did you experience a negative event?”	Dichotomous scale: yes/no
Situation cues: Activities	Response options adapted from Rauthmann et al. (2014)	1	“What have you just done?”	Multiple response options (e.g., sleeping)
Situation cues: Location	Response options adapted from Rauthmann et al. (2014)	1	“Where were you during this activity?”	Multiple response options (e.g., at home)
Situation cues: People	Response options adapted from Rauthmann et al. (2014)	1	“Who else was involved in this activity?”	Multiple response options (e.g., partner)
Daily-diary assessment
Affect	I-PANAS-SF (Janke & Glöckner-Rist, 2012; Thompson, 2007)	10	“Active”	5-point response scale
Personality states	Items based on Bleidorn (2009)	10	“Please rate the extent to which these adjectives described you today”	Bipolar scale ranging, for example, from strained to relaxed
Daily events	—	5	“Did you experience a stressful event today?”	Five items related to the occurrence, timing, and description of daily events
Perception of daily events	DEQ (Haehner, Fliedner, et al., 2024)	8	“The event was positive”	Rated on a 5-point scale from does not apply at all to totally applies
Weekly assessment
Big five traits	BFI-2-S (Rammstedt et al., 2020; Soto & John, 2017)	30	“I am someone who is outgoing, sociable”
	Big five triplets (Wetzel & Frick, 2020)	20	“I get stressed out easily.”	Participants select the statement that describes them best
			“I have little to say.”
			“I distrust people.”
Self-esteem	SISES (Robins et al., 2001)	1	“I had high self-esteem”
Self-efficacy	—	1	“I could rely on my ability to solve problems myself”
Self-control	—	1	“I had high self-control”
Self-concept	—	1	“I had a clear sense of who I am and what I want”
Life satisfaction	—	1	“I was satisfied with my life”
Depression and anxiety	PROMIS (Pilkonis et al., 2011)	2	“I feel fearful”	Only two items from PROMIS scale used
Perceived stress	PSS-4 (Cohen et al., 1983; Klein et al., 2016)	4	“How often have you felt that you were unable to control the important things in your life?”
Perception of change	—	5	“Extraversion: An extraverted person is characterized by being active, assertive, energetic, enthusiastic, open-minded and talkative.”	Assessed a 3-point response scale ranging from I have become more like this trait to I have become less like this trait

Note. Unless not otherwise indicated, constructs of the weekly assessment are assessed on a 5-point scale.

Mobile sensing

In addition to the questionnaires, participants are given the opportunity to activate mobile sensing. If the necessary permissions are provided, data are collected directly after participants complete interactions in the m-Path app (i.e., no continuous assessment). The types of data collected are: GPS location, app usage, step count, noise level, and nearby Bluetooth devices. Participants can agree to the collection of all types, some types, or no mobile sensing data at all.

Intervention phase and treatment groups

At the beginning of the intervention phase, participants download the CHILL Area applet in m-Path – an app within the m-Path app. This applet accompanies participants’ navigation through the intervention phase (see Figure 2), with some intervention strategies, such as Psychoeducation or Structural Support, being continuously available in the CHILL Area. However, most intervention strategies are prompted at specific times throughout the day, with different intervention strategies being implemented across the four core treatment groups (TGs 1 to 4; see Table 2). This randomization of intervention strategies across TGs allows us to investigate which intervention strategies are most effective in changing neuroticism. TG 1 only receives trait-level interventions that are continuously accessible in the CHILL Area, such as Psychoeducation and Information Feedback. This group may thus be seen as an active control group that allows to evaluate how minimal intervention efforts are related to personality changes. TG 2 receives trait-level and habit-level interventions. TG 2 is further split into two subgroups (TG 2a and TG 2b) to distinguish cognitive and behavioral intervention strategies. Comparing these two subgroups allows researchers to evaluate whether behavioral or cognitive strategies are more effective in changing neuroticism. TG 3 receives trait-level and state-level interventions. Finally, TG 4 receives intervention strategies that target all levels of neuroticism. TG 4 is again split into two subgroups (TG 4a and TG 4b) to distinguish between state-level intervention strategies that require participants to perform purely behavioral activities (e.g., Behavioral Activation) and strategies that also target cognitive and/or affective aspects (e.g., Mindfulness and Meditation Training).

Figure 2.

Illustration of the CHILL Area applet in m-Path. At the beginning of the intervention phase, participants download the CHILL Area applet as their intervention companion. The CHILL Area includes, for example, psychoeducational content on neuroticism and personality development.

Intervention strategies

In total, 17 different intervention strategies are examined in the CHILL Study. In the following sections, we describe the implementation of each intervention strategy in more detail. Further details on the taxonomy of intervention strategies can be retrieved from https://personalitychange.shinyapps.io/NeurTaxonomyShinyApp/.

Trait-level interventions and continuously presented strategies

Information feedback

Information Feedback is an intervention strategy that primarily addresses cognitive aspects of neuroticism. This strategy is continuously presented to participants with the goal of increasing their intervention adherence (e.g., Biesen et al., 2024). For example, after completing a questionnaire, participants receive a message stating the amount of compensation they have earned so far. In addition, participants can see a progress bar on their app homepage, allowing them to track how many questionnaires and tasks they have completed and how many remain for each day.

Psychoeducation

Psychoeducation (Walsh, 2009) is an intervention strategy that primarily addresses cognitive aspects of neuroticism. Information on the definition of personality, the costs of neuroticism, and personality development are sent out as notifications during the first intervention week and subsequently provided in the CHILL Area (based, for example, on Bleidorn et al., 2022).

Structural or practical support

Structural or Practical Support is an intervention strategy that addresses cognitive and behavioral aspects of neuroticism. Information on mental health services and counselling are continuously accessible in the CHILL Area.

Habit-level interventions

Habit-level interventions strategies are provided to TGs 2 and 4. These intervention strategies are presented twice per week.

Cognitive restructuring

Cognitive Restructuring (Clark, 2013) is an intervention strategy that primarily addresses cognitive aspects of neuroticism. When receiving this strategy, participants are provided with information on cognitive biases and asked to describe one negative thought that they would like to change. Then, they are guided to reflect on how cognitive biases may have influenced this thought and prompted to develop a more adaptive alternative.

Goal setting

Goal Setting is an intervention strategy that primarily addresses cognitive aspects of neuroticism. Participants receive instructions on how to formulate specific, concrete, and realistic goals to become less neurotic (e.g., Hudson & Fraley, 2015). They are then guided to develop one “if-then” plan for the following days (e.g., “When I come back from work, I will put on my running shoes and go for a 20 min run”).

Motivated reflection

Motivated Reflection is an intervention strategy that primarily addresses cognitive aspects of neuroticism (e.g., Stieger et al., 2021). When receiving this strategy, participants are guided to reflect on their experiences of the day, week, or on how they have changed since starting the study (e.g., “How have you changed since you started the CHILL Study? Write down the areas in which you have or have not noticed a change. How does this make you feel?”).

Positive reinforcement

Positive Reinforcement is an intervention strategy that primarily addresses behavioral aspects of neuroticism. This strategy is incorporated as a companion strategy to the other intervention strategies (e.g., Yeo et al., 2016). After completing a task, participants receive encouraging messages (e.g., “Great! Every step outside your comfort zone is a great success!”).

Resource identification

Resource Identification (Wilson, 2017) is an intervention strategy that primarily addresses cognitive aspects of neuroticism. When receiving this strategy, participants are guided to reflect on their resources in different life domains (e.g., “Think about and research what resources are available in your environment. For example, find out what psychological support services are available in your area or online. You can also look for group meetings on topics that concern you. After you have looked at your collected resources again, we would like to ask you to briefly consider which of them you can use at the moment”).

Skill building via modeling

Skill Building via Modeling (Ladd & Mize, 1983) is an intervention strategy that addresses cognitive and behavioral aspects of neuroticism. When receiving this strategy, participants are guided to reflect on how emotionally stable people in their environment deal with stressful situations (e.g., “Think of a person who is particularly good at dealing with stress. What behaviors do you observe in them? Describe how you could apply one of these behaviors in your everyday life”).

Skill building via practice

Skill Building via Practice (Coleman et al., 1993; Lyman et al., 2014) is an intervention strategy that primarily addresses behavioral aspects of neuroticism. Participants receive information on skills that are relevant to behave in a less neurotic way (e.g., emotion regulation, stress management, problem solving) and are then guided to implement these skills in their everyday life (e.g., “Imagine your best friend tells you about difficulties in their relationship. How would you use active listening? Give an example of how you – in your imagination – paraphrase, pay attention, and summarize at the end”).

State-level interventions

State-level interventions strategies are provided to TGs 3 and 4. These intervention strategies are presented three times per week.

Behavioral activation

Behavioral Activation (Dimidjian et al., 2011) is an intervention strategy that primarily addresses behavioral aspects of neuroticism. When receiving this strategy, participants select one of three suggestions for behavioral activation throughout the day (e.g., “Do something today that you usually avoid out of anxiety”) and reflect on their success in implementing this activity in the evening (i.e., whether they were able to implement it and how they liked it).

Discrepancy awareness

Discrepancy Awareness (Allemand & Flückiger, 2017) is an intervention strategy that primarily addresses cognitive aspects of neuroticism. When receiving this strategy, participants are guided to reflect on differences between their actual and ideal self (e.g., “What does a typical day in your life look like when you have achieved emotional stability? Which activities, habits and relationships contribute to your inner peace?”). Furthermore, participants receive feedback on their neuroticism scores and changes in their neuroticism levels once per week.

Emotional support

Emotional Support is an intervention strategy that addresses all aspects of neuroticism (e.g., Blossom et al., 2020). When receiving this strategy, participants select one of three suggestions to activate their social network (e.g., “Text a friend today and tell them about your day or about something that has moved you”). In the evening, participants then reflect on their success in implementing and experience of this suggestion.

Exposure to humor

Exposure to Humor is an intervention strategy that addresses affective and cognitive aspects of neuroticism (e.g., Berk & Nanda, 1998). This strategy is incorporated as a companion strategy to the Skill-Building via Modeling component by including humoristic elements in the videos. An example video can be found here.

Mindfulness and meditation training

Mindfulness and Meditation Training (Wielgosz et al., 2019) is an intervention strategy that addresses all aspects of neuroticism. Participants receive guided meditation or mindfulness tasks, such as an audio file with a meditation exercise. In the evening, participants then reflect on their success in implementing and experience of this task.

Physical activity

Physical activity is an intervention strategy that primarily addresses behavioral aspects of neuroticism (e.g., Dinas et al., 2011). When receiving this strategy, participants select one of three suggestions for physical activity in the morning (e.g., “Go for a walk for at least an hour today”). In the evening, participants then rate their success in implementing this strategy and whether they enjoyed it.

Relaxation

Relaxation (Jorm et al., 2008) is an intervention strategy that addresses affective and behavioral aspects of neuroticism. When receiving this strategy, participants receive a video with a guided relaxation exercise. In the evening, participants then reflect on their success in implementing and experience of this task.

Expected outcomes, analysis approaches, and practical implications

Data from the CHILL Study can be used to examine the effects of a digital smartphone intervention on self-reported, peer-reported, and implicitly assessed neuroticism, while also allowing researchers to explore spill-over effects to other constructs and investigate long-term intervention effects. Other critical questions that can be addressed with the CHILL Study concern the optimal timing and frequency of intervention components. For example, do more interventions prompts lead to more pronounced neuroticism changes? At what level may participants be overburdened with intervention prompts? Is there an optimal time point or context to prompt intervention strategies? Furthermore, the CHILL Study will provide insights into whether intervention strategies differ in their efficacy in changing neuroticism: Do different intervention approaches have comparable effect sizes (similar to the dodo bird effect; Wampold et al., 1997) or does intervention efficacy vary systematically depending on the targeted level and aspect of neuroticism? Finally, the daily diary and ESM assessments allow researchers to investigate whether and how experiences and dynamics in daily life may change in response to different types of intervention strategies.

To examine these effects, multilevel modeling and longitudinal structural equation models can be used to deal with the nested data structure. Another analysis strategy that is particularly suitable to test the efficacy of different intervention strategies in the CHILL Study involves the use of computational models (Farrell & Lewandowsky, 2010) that capture how a certain input (e.g., an intervention strategy) can alter a network of states (e.g., negative affect) to impart change in an output (e.g., neuroticism; Revelle & Condon, 2015). In contrast to variable-based models, like structural equation models, that examine covariations among variables to infer causality, computational models require researchers to articulate and test a particular conceptual model of the phenomenon under study. This requires scholars to specify assumptions that are oftentimes left implicit, such as the anticipated timescale of an effect (i.e., amount of time when a meaningful change in neuroticism can be expected, such as days, weeks, or months) and the shape of a change (e.g., gradual change vs. non-linear, stepwise effects). Thus, a core advantage of using computational models is that these models not only indicate if there is an effect, but also provide information about how that effect unfolds over time and across contexts (Riley et al., 2016). As the CHILL Study has assessments of relevant inputs and state variables across multiple timescales, its data can be combined with a computational modeling approach to advance our theoretical understanding of neuroticism change.

The practical utility of a smartphone intervention like the one examined in the CHILL Study is evident in the critical relevance of neuroticism for people’s mental health and well-being (Beck & Jackson, 2022; Cuijpers et al., 2010; Soto, 2019). Compared to psychotherapy which addresses related problems like depression and anxiety, digital smartphone interventions offer flexible and cost-effective intervention approaches that can easily be used by many people (Hardeman et al., 2019; Nahum-Shani et al., 2015). However, smartphone approaches also have important limitations, including being limited to people who have a sufficient intrinsic motivation to decrease their neuroticism. As such, the intervention tested in the CHILL Study may fill a different gap in the mental health space than psychotherapeutic approaches. Investigating the efficacy of an intervention like the one implemented in the CHILL Study is important to have efficacious treatment tools across the entire spectrum, from subclinical to clinical neuroticism.

Conclusion

As many people desire to become less neurotic (Baranski et al., 2021; Hudson & Fraley, 2016), developing an effective neuroticism intervention that can effectively reduce neuroticism in the general public is of critical practical relevance (Bleidorn et al., 2019). The CHILL Study aims to evaluate the effects of a digital intervention to promote neuroticism change. It moves beyond existing research on volitional personality change as it allows to distinguish the effects of different intervention strategies and provides insights into individual differences in intervention-induced neuroticism change. As such, the CHILL Study may facilitate the development of a comprehensive theory of neuroticism change (Bleidorn, 2024).

Supplemental Material

Supplemental Material - A smartphone-based intervention to decrease neuroticism: Protocol of the CHILL Study

Supplemental Material for A smartphone-based intervention to decrease neuroticism: Protocol of the CHILL Study by Peter Haehner, Amanda J. Wright, Rosalie Andrae, Till Lubczyk, Leyla A Rosero Betancourt, Christopher J. Hopwood and Wiebke Bleidorn in Personality Science

Footnotes

Author note

Dr Carolyn MacCann was the handling editor.

Acknowledgements

We would like to thank Mathias Allemand for sharing his insights and materials on personality interventions with us.

Author contributions

Peter Haehner: Conceptualization, Investigation, Methodology, Project administration, Writing – original draft, Writing - Review & Editing.

Amanda J Wright: Conceptualization, Investigation, Methodology, Project administration, Writing - Review & Editing.

Rosalie Andrae: Conceptualization, Investigation, Methodology, Project administration, Writing - Review & Editing.

Till Lubczyk: Conceptualization, Investigation, Methodology, Project administration, Writing - Review & Editing.

Leyla A Rosero Betancourt: Conceptualization, Investigation, Methodology, Writing - Review & Editing.

Christopher J Hopwood: Conceptualization, Investigation, Methodology, Supervision, Writing - Review & Editing.

Wiebke Bleidorn: Conceptualization, Investigation, Methodology, Project administration, Supervision, Funding acquisition, Writing - Review & Editing.

Declaration of conflicting interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Wiebke Bleidorn and Christopher J. Hopwood are members of the journal’s editorial board.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The data collection of the CHILL Study was funded by an SNSF Consolidator Grants awarded to Wiebke Bleidorn [Grant number 213696].

ORCID iD

Peter Haehner

Data accessibility statement

The design of the CHILL Study was preregistered at . After completing the data collection, we will publish an anonymized dataset on OSF.

Supplemental material

Supplemental material for this article is available online. Depending on the article type, these usually include a Transparency Checklist, a Transparent Peer Review File, and optional materials from the authors.

Note

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