Differences Between Consistent and Inconsistent Handedness Remain Consistently Interesting: Ten Years of Research on the Consistency of Handedness With the Edinburgh Handedness Inventory

Abstract

Almost 10 years ago Prichard et al. (2013) published a literature review on consistency of handedness. They described how consistency of handedness, typically measured by the Edinburgh Handedness Inventory (EHI), predicted outcomes in memory and decision-making paradigms better than direction of handedness. In the last 10 years, new research has extended these findings and added new theoretical perspectives. The goal of this short form review is to highlight and summarize some of these more intriguing findings and to encourage researchers in the fields of memory and decision making to incorporate handedness as a variable in future research.

Introduction

In 2013, Prichard et al. (2013) published a short literature review summarizing evidence that consistency of handedness, which refers to how strong one’s preference is for their dominant hand, is a statistically more powerful predictor of individual differences than direction of handedness, which refers to whether a person is left or right handed. It has been 10 years since that original paper was written. Since that time, a series of studies using the Edinburgh Handedness Inventory (Oldfield, 1971), hereafter referred to as the EHI, to measure handedness have expanded both the empirical knowledge of the relationship between consistency of handedness and memory and the complexity of the theoretical ideas surrounding this relationship. The goal of the present paper is to provide an update on some of the theoretical and empirical findings that have emerged on the topic of consistency of handedness since that time, with a special focus on research that uses the EHI to divide participants into consistently and inconsistently handed groupings and then investigates cognitive and perceptual differences between the groups.

Because this paper is intended to serve as a follow-up to a previous short literature review, some background on the Prichard et al. (2013) manuscript is in order. The idea that handedness can be divided into left or right handedness or into groupings based on degree or strength of handedness long predates the 2013 paper. For example, it is possible to find research by Annett referring to left-handers, right -handers, and mixed-handers going back at least as far as the 1970s (e.g. Annett, 1970; Annett, 1978). Around the same time, Oldfield created an assessment of handedness called the Edinburgh Handedness Inventory. It listed the 10-following common manual tasks: writing, drawing, throwing, scissors, toothbrush, knife, spoon, broom, striking match, opening box (lid). In the original version, each task had two columns: a “left column” and a “right column.” Participants were to put two plus marks in a column if they exclusively preferred one hand for a task (e.g. two plus marks in the right column if they exclusively preferred the right hand) and one plus mark in each column if they were indifferent. The original purpose was to measure both direction and degree of handedness. Prichard et al. (2013) reviewed a series of studies comparing individuals divided into strong handers and mixed handers on the basis of the EHI. They found a consistent pattern in which degree or strength of handedness would frequently predict individual differences on psychological tasks even when direction of handedness did not.

There are a few caveats to note. First, here we use the nomenclature inconsistent and consistent as opposed to weak and strong, because often the “mixed-handed” grouping contains participants who still have a strong hand preference, but use their non-dominant hand for at least some tasks. We consider truly “mixed-handed” individuals to be closer to true ambidexterity than the broader grouping “inconsistent,” which includes mixed-handers and people with a general lateralized preference who have a strong preference for their non-dominant hand on two or three common tasks. Second, much of the research Prichard et al. (2013) cited compared inconsistent and consistent right handers but excluded left handers. That is important to note because this approach limits the generalizability of some findings to right handers only. Finally, much of the research mentioned here, and in the Prichard, et al. (2013) paper, use versions of the EHI that are similar to, but somewhat modified from the original Oldfield version (e.g. Christman et al., 2015). Often these versions maintained most of the original items, but dropped “broom” and reconfigured the scoring system to range from −100 (left preference for all tasks) to +100 (right preference for all tasks). In the studies reviewed for this paper, consistency of handedness, was defined by perfoming a median split on the absolute scores of the EHI. Participants whose absolute scores were in the upper half of the distribution were defined as consistently handed. People who scored in the lower half were defined as inconsistently handed.

At the outset of the manuscript, we wish to acknowledge several points. Most importantly, this manuscript is not meant to serve as a comprehensive review of all handedness literature or all literature using the EHI, but, rather, as a micro-review of a specific thread of literature that has built upon research originally reviewed by Prichard et al. (2013). We do not aim to mislead by excluding other literature, but to highlight a relatively narrow set of findings which we believe could be of broader interest once it has been aggregated and synthesized. Secondly, we wish to acknowledge the importance of replication. Ultimately, the theoretical models presented here will live or die by the results of experimentation.

As the original manuscript to which we are responding contained sections on memory, cognitive flexibility, and miscellaneous findings, we follow a similar format here. We first revisit findings related to memory and consistency of handedness and end the section with a theoretical model of the interrelationship between handedness, saccadic eye-movements, and recall. We next provide a section on handedness and decision-making/belief updating and summarize a recently proposed dual process model (Jasper, et al., 2021).

Recent Findings Related to Handedness and Memory

The general pattern of results regarding consistency of handedness and its relationship to memory, as reported in the Prichard et al. (2013) review, has been that inconsistent handers tend to perform better on tests of memory than consistent handers. But, how have these results held up over time? An intriguing early contribution of the last 10 years came from a set of null results. While we caution that a null result is not exactly a disconfirmation of a theory, a replicated null result may still be informative. Sahu and Christman (2014) published a set of studies comparing inconsistent and consistent handers on two measures of prospective memory. In the first study, 105 participants completed a self-report measure of prospective and retrospective memory. In the first study, there were no self-reported differences in prospective or retrospective memory between consistent and inconsistent handers. However, self-report can be unreliable, and behavioral data is a stronger form of evidence. Study two therefore improved upon the design by using a behavioral task meant to directly measure prospective memory performance. This study again obtained no evidence of a handedness difference. The authors concluded that inconsistent handedness is associated with superior retrospective, but not prospective, episodic memory. Relatedly, Parker et al. (2017) reported no handedness differences in the number of specific future predictions that people generated, while still finding the typical inconsistent-handed advantage in number of autobiographical memories. Explanations include the possibilities that prospective memory relies more heavily on semantic than episodic memory, that retrospective memory is a right hemisphere lateralized process, and that retrospective memories contain spatiotemporal elements that are missing from prospective memories for future tasks to be completed. None of these explanations is mutually exclusive of the other, and it is possible that all three explanations are at play.

A second paper by Sahu et al., 2016 added further nuance to the field’s understanding of the relationship between handedness and memory with yet another null result. Two-hundred ten participants were given a paired associates task designed to test long term episodic memory and a digit span task meant to test working memory. As predicted and consistent with previous findings, both working memory capacity and inconsistent handednesswere were associated with better performance on the episodic memory task. However, there was no handedness difference on the digit span task, indicating that working memory capacity and inconsistent-handedness both contributed independently to superior episodic memory. This finding was consistent with the idea that long term episodic retrieval is a more right lateralized task and that the inconsistent handers’ advantage on memory tasks is (i) not dependent on a working memory advantage and (ii) likely occurs at retrieval and not encoding.

Not all of the research advances on the relationship between handedness, as measured by the EHI, and memory have resulted from null results. Several interesting positive results have added to the literature as well. Prichard and Christman (2017) published a set of studies that replicated previous findings of better long-term memory for inconsistent handers, but they used paragraph level prose stimuli. Interestingly, the handedness effects accounted for variance above and beyond variance accounted for by other established memory effects observed in the paper, such as the effects of a retroactive interference condition in study one and the effect of context in study two. As with the working memory paper published by Sahu and Christman (2014), inconsistent handedness was shown to be a robust correlate of memory in addition to other factors. If anything, it suggests that every memory study should at least include handedness as a factor, even if it is not the main focus of the study.

In this vein, special attention should be paid to the relationship between handedness and saccadic bilateral eye movements. In 2003, a study produced evidence that bilateral eye movements enhanced memory for consistent but not inconsistent handers (Christman, et al., 2003). The authors proposed that eye movements enhanced interhemispheric communication via the corpus callosum. More recently, Qin et al. (2021) conducted a metanalysis of 22 papers investigating this effect in right handers going back to 2003. Empirically, the metanalysis agreed with the original findings of Christman et al. (2003). Horizontal eye movements seemed to enhance memory, but this effect was moderated by handedness, with consistent right handers benefiting most. However, a potential limitation was the lack of left handers in the analysis. Thus, we must be careful not to conflate inconsistent right handedness with inconsistent handedness in general. Indeed, future replications could benefit from including larger numbers of left handers where possible.

This is also an area in which recent applied research has contributed new information. For example, handedness was shown to have a moderating effect, with horizontal eye movements, on enhanced eyewitness recall (Kelley & Lyle, 2021). There was also a main effect of horizontal eye movements, without assessing for handedness, on improved autobiographical memory for older and younger adults (Parkin, et al., 2022). Both of these applied research findings suggested that bilateral eye movements may be useful for enhancing witness testimony and helping older adults remember past events.

While some studies have failed to replicate a memory improving effect of eye movements (e.g., Polden & Crawford, 2022), the metanalyses conducted by Qin et al. (2021) found moderate effect sizes across published studies (Cohen’s d = .45). There are several important things to note about these findings. First, academic psychology has a persistent pervasive bias towards accepting positive over null results (e.g., Scheel, et al., 2021), and this bias can lead to metaanalyses that overestimate the size of even a real effect. Second, it is often these inflated effect sizes from positive results that are then used to determine sample sizes in follow-on research. For example. Polden and Crawford (2022) used an estimated effect size of .495 (already larger than Qui et al.’s metaanalysis effect size of .45) to determine a sample size of sufficient statistical power. However, sufficient power has frequently been assumed to be .80, meaning that researchers were willing to accept up to a 20% chance that a real effect would not be detected. All of this is to say that the collective literature has been consistent in finding a real and robust effect of eye movements on memory. However, scientists engaged in future attempts to expand and replicate this research should be aware that the effect size is at best, moderate, meaning that larger sample sizes than previously used are warranted going forward.

A question still remains as to what underlies this effect of eye movements on memory. The original proposal by Christman et al. (2003) was that eye movements increased interhemispheric activation. If retrieval is primarily a right hemisphere process and inconsistent handers already have more access to their right hemispheres, then only consistent handers would benefit from eye movements. However, Lyle and Edlin (2015) called this retrieval hypothesis into question and proposed instead that saccadic eye movements increased top down attentional control by recruiting frontoparietal networks involved in attention. Is there some way to rectify these two hypotheses? Prunier et al.(2018) proposed that saccadic eye movements increased general cortical activation. They proposed that the relationship between activation and performance on a memory task should follow an inverted-U pattern in which activation increases memory up to a certain point, but then leads to decrements. They also proposed that inconsistent handers would reach their activation peak earlier, meaning that they would be less likely to be helped and might, in fact, be somewhat impaired by extra activation.

To test the cortical activation hypothesis, Prunier et al. (2018) assessed 316 participants memories of a list of 36 words, dividing participants by five levels of their grip strength (measued by their squeezing of hand dynamometers). Increased grip strength was associated with increased memory performance, but inconsistent handers peaked at a lower level of activation (operationalized as grip strength) than did consistent handers. This finding suggested that general resting activation for inconsistent handers might be higher than for consistent handers. Collectively, the authors of studies reviewed here have hypothesized that experimental tasks, such as bilateral eye movements and grip strength, increase general activation, but more direct experimental evidence between these tasks and cortical activation are needed. But, assuming that experimental evidence to date confirms the link between performing saccadic eye movements or gripping a dynamometer and brain activation, does this increase in brain activation mean more activation for the right hemisphere in consistent handers or more activation of the frontoparietal attentional control networks in consistent handers? Perhaps both the hemispheric-asymmetry and the attentional control hypotheses contain separate elements of the truth.

Prichard and Christman (2021) tested the effects of eye movements at encoding by manipulating the width of prose passages. Passages of material to be remembered were either ∼28 or ∼120 characters in length. The passages were the same across conditions meaning that participants in the narrow passage condition made more bilateral saccadic jumps while encoding the material. In this study, there was an interaction effect in which the consistent handers with more saccadic eye-movements at encoding performed worse at retrieval. At first, this may seem to be counterintuitive. Previous research showed that eye movements enhanced memory for consistent handers. But, this was the case after the materials had been studied. In other words, consistent handers encoded the material and later eye movements enhanced retrieval. In this newer case, the eye movements happened during encoding. The authors proposed that eye movements increased overall activation in consistent handers, including activation of right frontoparietal areas involved in episodic retrieval. However, because consistent handers are more lateralized, this increased right frontoparietal activation actually competed with the cognitive resources involved in encoding.

Taken together, a working hypothesis of the relationship between consistency of handedness and memory might look something like this: Inconsistent handers may have higher baseline levels of cortical activation than consistent handers. This could include activation of frontoparietal areas involved in attentional control and episodic recall. Eye movements enhance overall activation and increase activity in right frontoparietal areas involved in episodic recall. However, this increased overall activation competes for local cognitive resources and encoding in consistent handers is temporarily diminished as a price for recall improving. This hypothesis is still rudimentary, but it shows that theory is continuing to evolve. We highly encourage more researchers to incorporate handedness into their research on memory.

Decision-Making, Belief Updating, and Dual Processes

Differences between consistent and inconsistent handers also extend to many different decision-making tasks and behavioral domains. What were previously referred to as “miscellaneous handedness findings” in Prichard et al. (2013), have now been shown to be part of a reliable and common pattern in the empirical literature. Here, we established degree of handedness as an important individual difference factor for predicting participants’ susceptibility to regular decision-making biases like the sunk cost effect (Westfall et al., 2012), one’s level of belief updating (Jasper, Fournier & Christman, 2014) and framing sensitivity (Jasper, Kunzler, Prichard, et al., 2014).

Briefly, framing is the tendency for people to reverse decisions made under risk when the calculated expected value of the decision remains the same, but the wording of the decision is changed to either emphasize possible losses or possible gains (Tversky & Kahneman, 1981). In Jasper, Woolf, and Christman (2014), inconsistent handers were more affected by framing manipulations, and they updated their beliefs to new information, while consistent handers showed no sensitivity to the frames. In another study, Christman and Jasper (2014) found that handedness influenced risky decision-making between variants of the Asian disease task, where participants have classically been required to select between an option that might save “all” individuals in a group or “none” of them (for original scenario see Tversky & Kahneman, 1981). Importantly, while consistent handers showed framing effects in standard variants of the task (e.g., when selecting between options that risked saving either “all” or “none”), inconsistent handers showed larger framing effects in scenarios with less extreme outcomes (e.g., deciding between options that saved “most” or “some”). Sunk cost refers to the tendency for people to continue pouring resources into a costly endeavor because they have already committed resources to it and ceasing with the endeavor would be perceived as a loss of the previously expended resources (Thaler, 1980). Examples might be spending money on a declining investment or sending more troops and equipment on a military mission that has failed up to the present. Westfall et al. (2012) found that inconsistent handers generally showed a greater sunk cost effect because they experienced more aversion to loss. However, in experiment four, these investigators were able to reverse the result by emphasizing to participants that continuing a costly endeavor would lead to more long term loss.

The sheer number of handedness effects across decision making domains, as well as methodological differences regarding the measurement and classification of handedness has made a unified explanation of these findings challenging, besides appealing to the previously mentioned differences in brain lateralization (i.e., interhemispheric integration and baseline right hemisphere activity) observed between consistent and inconsistent handers. In addition, the fact that not all handedness findings have been replicated (see Gyimesi et al., 2019) adds to these difficulties. Jasper et al. (2021) presented a more comprehensive explanation that couches past findings within the context of dual process models, arguing that while integration facilitates functioning of certain dual processes, others rely on increased functional independence.

Consider the case of belief updating, in which converging lines of evidence strongly suggest that mechanisms in the right hemisphere (which update beliefs) inform belief maintenance mechanisms in the left (e.g., Christman, 2022; Doricchi, et al., 2022). In such cases, factors that increase interhemispheric communication (like inconsistent handedness) should lead to increased integration of lateralized dual processes and thus, more belief updating (Christman et al., 2008). Inconsistent handers should, and do in fact, drive these effects, by showing increased belief updating compared to consistent handers (Jasper, Woolf, & Christman, 2014). This pattern reverses, however, for other lateralized dual processes that theoretically rely on increased functional independence. For example, consider sensitivity to gains or losses, where withdrawal mechanisms in the right hemisphere coding for risk aversion (loss sensitivity) stand in contrast with left hemisphere mechanisms that code for approach (gain sensitivity). Here, factors that increase the independence of these processes (like consistent handedness) ought to drive the effects. That is, consistent handers should be more sensitive to the framing of choices as gains or losses than inconsistent handers (for such a result, see Stein, 2012).

While Prichard et al. (2013) previously argued that degree of handedness should be added as a routine subject variable across many domains of research, based on the observations above, this is particularly true of studies that investigate cognitions or behaviors that are instantiated by dual processes. As argued by Jasper et al. (2021), degree of handedness often interacts with other variables in such cases, as (i) consistent handers show larger effects when experimental manipulations target independent dual processes and (ii) that inconsistent handers drive effects when manipulations target integrated dual processes. Importantly, this theoretical account relies upon (but also extends) previous explanations that cited differences in brain lateralization as the primary factor underlying the effects of handedness on decision making.

Notably, within this context, the importance of these differences (e.g., corpus callosum size and right hemisphere activity) are understood, based on how they influence the integration or independence of dual processes. While this is a parsimonious accounting of the underlying causes driving handedness effects, it is tentative, and additional data is needed to evaluate this theory. Such data might come from investigations that explicitly and intentionally test the performance of consistent and inconsistent handers on tasks that rely on integration or the independence of dual processes. Additionally, other research designs might search for mechanisms that can account for these effects. Such designs need not be explicitly geared toward handedness per se but might simply investigate the effect of lateralization on the integration or independence of cognitive processes in typical or clinical populations.

While research on handedness and its effect on decision making continues to move forward, future studies would do well to investigate explicitly for interactions between handedness and decision making in the context of dual process models. Indeed, given the popularity of such models, e.g., moral judgment (Greene et al., 2009) and risky decision making (Diederich & Trueblood, 2018), there is no shortage of opportunities for such research. In this vein, such studies stand to offer insights in their ability to evaluate and extend this new idea.

Concluding Remarks and Limitations

While not exhaustive, we hope this review provides a succinct update of research using the EHI to measure consistency of handedness. We also hope other researchers will consider incorporating this handedness measure as they explore topics such as cognition and decision making.

There are several limitations to the current analysis worth mentioning. First, this has been a short review that summarizes a relatively narrow subset of the handedness literature. The goal here was to highlight findings from within this narrow literature, but we hope it will be useful for researchers looking to take on a more comprehensive account of human handedness. Second, as mentioned in the article, there is a hypothesized connection between handedness, eye movements, grip strength, and brain activation. However, more direct tests of these hypotheses need to be carried out using brain imaging technologies. Finally, some of the aforementioned research used slightly different criteria for classifying consistent and inconsistent handers. While many studies mentioned here and by Prichard et al. (2013) used an absolute EHI score of 80 as a cutoff, some others, such as the study reported by Prichard and Christman (2017), used different cutoffs. While they justified their research design decision by stating that used a median cutoff to achieve equal sample sizes for the two conditions, we acknowledge that a move towards more standardization will likely result in greater replicability and cogency across the discipline.