ABSTRACTHuman handedness has long been classified in binary terms, yet such categorical distinctions increasingly appear at odds with both conceptual and empirical advances in psychological research. This commentary examines some of the challenges facing laterality researchers in reconceptualizing handedness. First, it considers the practical tension involved in measuring handedness as a continuous variable: though this affords greater precision it imposes a time and attentional burden on participants, particularly in studies where handedness is not a primary interest. Second, it explores how technology is changing handedness, with the decline of handwriting and the rise of bimanual digital devices challenging the validity of the most commonly used short measure of handedness: writing hand. Third, it argues that rather than abandoning categorical classification altogether, researchers could consider a more nuanced model that offers greater ability to distinguish differences in handedness than binary conceptualizations, whilst still retaining the intuitive benefits and ease of measurement of the categorical approach. By refining how we categorize handedness rather than abandoning categorization altogether, researchers can recognize both the complexity of the construct and the realities of empirical work.

ABSTRACTThe healthy brain shows a hemispheric lateralization biasing attention to the left hemifield, a phenomenon known as pseudoneglect. It is currently still not clear whether this bias can be modulated by training. Football officials represent an ideal population for investigating training effects on spatial attention, given that referees deploy attention across the field in all directions, whereas assistant referees monitor the guidelines back and forth only and allocate attention mostly to the left direction. Here, we assessed attentional biases from referees and assistants by administering a standard line bisection task. Results documented individual differences in bisection errors, with a prevalence, as expected, of errors signalling left attentional biases (i.e., pseudoneglect). Interestingly, this left bias decreased as a function of months of expertise for the referees group, whilst it remained essentially constant for the assistants group. This finding represents evidence that prolonged training in allocating attention to both visual hemifields can counteract the left attentional bias typical of most individuals.

Imitation deficits after brain damage are commonly assessed with meaningless hand positions (HP) and finger configurations (FP). Previous studies on unilateral brain damage and callosotomy individuals revealed left-hemispheric specializations for HP and bilateral representation for FP, with a right-hemispheric dominance. This study examined hemispheric specialization for HP and FP imitation, as well as FP subtypes, in five hemispherectomy individuals (n = 3 with a remaining left (rLH); n = 2 with a remaining right hemisphere (rRH)). Imitation performance was analysed with the NEUROGES®-ELAN system, considering error types and reaction times. Bayesian analyses revealed no group differences in performance scores, error types and reaction times for HP or FP imitation. The qualitative analysis of error types revealed group differences, particularly for FP imitation, with rRH individuals showing predominantly self-corrections, whereas rLH individuals showed diverse error patterns (e.g., position errors). However, considerable interindividual variability was foremost observed across imitation tasks and subtypes. Overall, the findings do not support a strict hemispheric specialization for HP and FP imitation following hemispherectomy. Rather, they highlight the importance of individual variability.

Maternal stress is known to influence neurodevelopmental trajectories in offspring, potentially affecting lateralized behaviours. In this study, we investigated the effect of pre- and postnatal maternal stress on paw preference in Ctenomys talarum, a wild subterranean rodent species that exhibits behavioural lateralization while feeding. Pregnant and lactating females were exposed to predatory stress and offspring later evaluated for paw use while manipulating food items. Similar to previous work, most offspring displayed a significant left-paw preference for carrying the food items. Results of analysis showed that maternal stress changes the relative distribution of paw use during feeding, slightly increasing the proportion of use of the right paw to take food to the mouth. This study contributes to the emerging body of evidence linking stress to modifications in lateralized behaviour and highlights C. talarum as a valuable wild model for investigating the ecological and neurobiological implications of developmental stress and behavioural asymmetries.

Facial appearance plays a key role in social and political judgments, yet the mechanisms linking facial pose and emotional expression remain unclear. One robust phenomenon is a left-cheek posing bias, often attributed to asymmetries in emotional expression. The current study examined this issue using facial photographs of all candidates (N = 1,107) in a Japanese national election. Head orientation and smile intensity were estimated using image-based measures. A reliable left-cheek bias was observed, with a stronger bias among conservative politicians. Left-cheek posing was not associated with smiling. However, among conservatives, reduced smiling was associated with an increased likelihood of right-cheek posing. These findings replicate posing asymmetries in a non-Western context and suggest that lateral posing asymmetries may be linked to the regulation of emotional expression.

Pseudoneglect, a subtle attentional bias toward the left visual field in neurologically healthy individuals, can be measured with a split-screen method in which a bright stimulus in one hemifield elicits stronger pupil constriction when it appears on the left than on the right (Strauch, C., Romein, C., Naber, M., Van der Stigchel, S., & Ten Brink, A. F. (2022). The orienting response drives pseudoneglect-Evidence from an objective pupillometric method. Cortex, 151, 259-271. https://doi.org/10.1016/j.cortex.2022.03.006). Replications of this method have reported mixed results, including a large Registered Report that replicated the effect with a small effect size (d = -0.31). Given strong left-field biases in the emotional chimeric face test, the present study (n = 53) combined this task with split-screen pupillometry to test whether emotional face processing amplifies pupillary pseudoneglect. Pupillary pseudoneglect was observed during control trials with non-faces (d = -0.36) and was stronger for emotional faces (d = -0.76), but this difference was not significant. A strong left-field bias was confirmed for emotional face judgements (d = -0.76), which did not correlate with pupillary bias (ρ = -0.26), and for a line bisection test (d = -0.47), which also did not correlate with pupillary pseudoneglect (ρ = -0.19). These data support the split-screen pupillometric method for measuring pseudoneglect and replicate a previously-noted pattern that leftward biases across tasks show little statistical relation to one another.

For more than a century, handedness has served as a canonical example of functional asymmetry in humans. Traditional perspectives have largely treated handedness as a binary trait, classifying individuals as either right- or left-handed / non-right-handed, with only occasional recognition of a third category such as mixed-handedness. This binary categorization has become deeply entrenched and often remains unquestioned. A consequence of this dichotomous framework is a long-standing bias in the interpretation of findings related to left-handedness, which by some authors has been associated with negative outcomes in clinical research or even has been proposed to be a marker of decreased survival fitness. These interpretations largely reflect methodological limitations, as individuals who do not fit neatly into either category are frequently misclassified or overlooked. This commentary argues that mixed-handedness should be recognized as a central aspect of laterality research. Incorporating this category challenges the conventional dichotomy, addresses limitations of existing classification systems, and has important implications for understanding neurodevelopment, cognition, and clinical outcomes. In the following sections we will shortly line out empirical evidence supporting this view.

Rodents, particularly rats and mice, are among the principal animal models in basic and clinical neuroscience. In humans, behavioural laterality, such as handedness, has been shown to substantially influence outcomes in both experimental and clinical research. In contrast, behavioural laterality in rodents is rarely considered in published studies, despite its potential relevance for interpreting neurobiological findings. This review provides a comprehensive overview of the significance of behavioural laterality in basic and clinical neuroscience, integrating evidence from human and rodent research. Current approaches to assessing laterality in rodents are summarized and contrasted with established methods used in humans, highlighting key methodological and conceptual differences. Systematic inclusion of behavioural laterality measures appears essential to fully leverage the explanatory power of rodent models for understanding brain organization and its relevance to neurological and psychiatric disorders. Moreover, progress in translational laterality research requires a complementary reverse-translational strategy in which insights from rodent models inform targeted investigations in humans. To advance this approach, a structured framework for empirical research in humans is proposed, derived from key findings and methodological strategies in rodent studies. This framework is intended to strengthen conceptual integration between animal and human research, aiming to improve the translational impact of laterality research.

Most people have a strong preference to use one hand over the other for complex motor tasks, and are more skilled at doing so, a phenomenon known as handedness. Is there also handedness for the sense of touch? This paper reports a systematic review and meta-analysis of studies that have compared tactile spatial acuity between the two hands in healthy humans. A total of 107 samples from 85 papers (total N = 4949) were identified from searches of the PubMed and Scopus databases in September 2024. Potential moderators of such differences were also explored, including psychophysical task, age, and geographic location. Overall, there was no evidence for systematic differences in tactile acuity between the two hands (Hedges's g = -0.021, 95% CI [-0.070, 0.028]), nor of any effect of potential moderating variables. There was, however, substantial heterogeneity in the results. The implications of these findings are discussed in relation to the links between motor and somatosensory function.

Background: Hand preference is influenced by genetic, anatomical, and environmental factors, leading to societal biases that stigmatize left-handed individuals, especially in Arab and Muslim cultures. These biases create pressures for conformity and feelings of inadequacy among left-handed individuals, highlighting the need for a scale to measure this stigmatization in the MENA region. Methods: A set of 54 items assessing left-handedness stigmatization was evaluated by seven experts, resulting in 24 items. An Exploratory Factor Analysis (EFA) with 266 participants identified 17 items clustering into three factors: "Stereotypes against left-handedness," "Right-handed normative pressure," and "Left-handed acceptance." Confirmatory Factor Analysis (CFA) was conducted with 423 participants. Results: The scale demonstrated good reliability (Cronbach's alpha = 0.78). The EFA and CFA provided good fits (RMSEA = 0.04, p = 0.002 and RMSEA = 0.0638, p < 0.001, respectively). Conclusion: The Left-Handed Stigmatization Scale for MENA (LHSS-MENA) is a good tool for assessing societal attitudes toward left-handed individuals, effectively capturing the nuances of stigma and societal pressures with reliable psychometric properties. KEYWORDS: Stigma; MENA region; Left-handedness; Scale; Factor Analyses.

An analysis of hand use by character role (protagonist princesses versus antagonist villains) from all 13 Disney "Princess" movies was conducted. Over time, protagonists became less right-handed than antagonists, with this relationship primarily occurring after 1992. Overall, protagonists became less right-handed over the past ∼80 years, while antagonists' handedness remained stable. Hand use in characters was much less right-handed than in the human population and, contrary to the cultural and historical association between left-handedness and evil, antagonists now tend to be more strongly right-handed than protagonists.

Public Significance StatementWe show that the brain's ability to recognize words and faces is driven by a top-down process that involves directing our attention to enhance recognition, such that attention benefits recognition of real words and faces, but not rejection of made-up words and inverted faces. Likewise, the brain's hemispheric advantages (left hemisphere specialized for words, right hemisphere for faces) are larger for recognition of real stimuli vs. rejection of false ones. These findings clarify that hemispheric advantages, whether innate (e.g., left hemisphere language superiority) or "on-the-fly" (directing attention to one side of space) apply mainly to real recognizable stimuli, and therefore involve a top-down processing mechanism.

Hemispheric asymmetry is a defining feature of the human brain, yet how weak early left-right biases develop into stable hemispheric specialization remains unclear. Although molecular mechanisms establish initial embryonic asymmetries, a mechanistic explanation linking these early biases to persistent hemispheric differences has been lacking. Here we introduce a minimal dynamical model that isolates the developmental conditions under which small left-right asymmetries can be amplified and stabilized. Each hemisphere is represented by a continuous maturation variable, with reciprocal interhemispheric interactions modelled as nonlinear inhibitory coupling. Analytical stability analysis shows that hemispheric differentiation emerges when the symmetric equilibrium becomes unstable, allowing asymmetry to arise as a dynamical outcome of coupled development rather than explicit hemisphere-specific programming. Numerical simulations demonstrate that stable hemispheric asymmetry robustly emerges once interhemispheric coupling exceeds a critical threshold. Without intrinsic bias (ϵ = 0), symmetry breaking occurs through stochastic amplification, producing left- and right-dominant outcomes with approximately equal probability. Introducing a weak intrinsic bias does not create asymmetry but biases the selection among available asymmetric states. These results provide a minimal dynamical bridge linking early embryonic asymmetry to later hemispheric specialization.

Reliable measures of language laterality are essential for understanding relationships between speech and other lateralized functions. In a previous study (Grant et al., 2023. Testing the relationship between lateralization on sequence-based motor tasks and language laterality using an online battery. Laterality, 28(1), 1-31), an online verbal visual half-field (vVHF) task assessed speech production laterality but showed limited reliability and validity. The present study conducted novel analyses linking this measure with functional transcranial Doppler ultrasound (fTCD) data collected from the same sample as part of a separate project (Parker et al., 2022. Inconsistent language lateralisation: Testing the dissociable language laterality hypothesis using behaviour and lateralised cerebral blood flow. Cortex, 154, 105-134). In total, 187 participants completed both the online battery and an in-person fTCD sentence-generation task. Results confirmed robust left lateralization for speech production across handedness groups. However, laterality indices from the vVHF task did not correlate with the fTCD measure, reinforcing concerns about the validity of the online paradigm. Replicating previous findings, no significant association was observed between categorical measures of speech and motor laterality. Exploratory analyses revealed a significant positive correlation between motor laterality indices and fTCD speech laterality.