Papadatou-Pastou and colleagues' rapid review maps 47 named tasks comparing the relative skill of the two hands. From a relational perspective, most tasks assess a person acting alone. This provides a baseline, but not a complete account of how manual asymmetry is expressed. Evidence that hand choice and attributed handedness vary with target animacy, communicative purpose, partner role, emotional context, or viewpoint raises a further question: can relative hand performance also change when the same or a matched action is embedded in a social context? We use relational handedness to denote systematic variation in hand choice or relative hand performance generated or modulated by an actor's relation to a partner or observer, given a social goal and spatial arrangement. Attributed handedness addresses the neighboring question of how the observer-agent relation shapes perceived manual laterality and helps identify relational variables to manipulate. Relational handedness is not a third score alongside preference and skill, but a testing condition potentially assessing their contextual stability. Future batteries could pair validated individual tasks with matched social and nonsocial conditions and test the hand-by-condition interaction. This would preserve the distinction between preference and skill while separating task noise from meaningful relational modulation.

The phenotype of human handedness is likely to be a product of the ecological conditions experienced by our ancestors, with different forms of handedness having different fitness costs and benefits. Human hands, limbs, and bodies move in a coordinated fashion, so that hand lateralization is likely to have also entailed the evolution of limb and body lateralization. Consequently, we suggest that a measure of handedness should incorporate measures of limb lateralization. Measures of handedness that do not represent these differences may not capture the functional properties of an individual's hand use, or the types of handedness that were selected for in ancestral environments. The advantages of measuring lateral preference rather than skill are also highlighted. Finally, we suggest that to determine categories from continuously measured data, it may be better to use algorithmic analysis techniques, such as cluster analysis, latent class analysis, or dual Weibull mixture models. These reduce researcher degrees of freedom, while being able to capture the dimensional and categorical nature of handedness. Ultimately, methods to determine handedness should be suited to the specific research question, and no single method may suit all circumstances.

Although language production is generally dominated by the left hemisphere, it remains uncertain whether the script format influences the degree of this lateralization. To examine this issue, verbal fluency was assessed in 96 right-handed native Chinese speakers using functional transcranial Doppler sonography (fTCD). The subjects were asked to perform four overt word-generation tasks with four combinations of cue format and fluency type: Pinyin phonological fluency, character phonological fluency, Pinyin semantic fluency, and character semantic fluency. Laterality indices were calculated as the mean left-minus-right middle cerebral artery blood-flow velocity difference across the complete prespecified period of interest from 4 to 14 s after cue onset. All four conditions showed significant left lateralization, with mean LIs of 3.05, 2.60, 2.76, and 2.26 for Pinyin phonological, character phonological, Pinyin semantic, and character semantic fluency, respectively; all ps < .001. Pinyin-cued tasks yielded greater lateralization than character-cued tasks, F(1, 95) = 117.61, p < .001, ηp[2] = .553, and phonological fluency yielded greater lateralization than semantic fluency, F(1, 95) = 32.81, p < .001, ηp[2] = .257. The results indicate that verbal fluency is left-dominant in Mandarin, while the script format modulates the degree of hemispheric dominance during lexical retrieval.

Human handedness shows a stable population-level bias toward right-hand preference, yet this pattern cannot be fully explained by genetic inheritance alone. In this commentary, we argue that niche construction theory provides a framework for understanding handedness as a family of dynamically constructed phenotypes that emerge through interactions between organisms and the lateralized niches they inherit and create. These niches include cultural norms, material artifacts, technological practices, and symbolic representations that can shape, stabilize, suppress, or reinterpret hand preference across development and history. From this perspective, handedness may seem to get "out of hand" when observations across these domains accumulate without sufficient theoretical constraint. Niche construction theory helps tighten this problem by situating such observations within inherited and constructed lateralized niches, suggesting that the question of how many handednesses exist depends on how those niches are individuated and studied. Rather than proposing a fixed universal taxonomy, we suggest that handedness is best understood as niche-specific, historically contingent, and shaped by multiple interacting inheritance systems.

This commentary evaluates the conceptual and methodological challenges in handedness research, focusing on the implications of prevailing measurement practices. Although handedness has traditionally been viewed as a binary trait (left vs. right), the widespread adoption of psychometric instruments has introduced a degree of continuity that may not accurately reflect its underlying biological structure. Scales, such as the Edinburgh Handedness Inventory, generate continuous scores that are frequently partitioned into arbitrary categories, leading to conceptual ambiguity and limited comparability across studies. The assumptions that hand preferences are inherently continuous and that handedness is a multidimensional trait with equally significant dimensions are challenged. Instead, it is argued that dimensions, such as the "degree" or "consistency" of hand use, may reflect contextual and probabilistic variation rather than independent latent factors. Drawing on analogies from biology and psychiatry, a hybrid structure is proposed, in which discrete latent categories coexist with graded phenotypic expression. To address this complexity, two less well-known methodological approaches are described: taxometric analysis, which identifies categorical latent structures, and fuzzy classification, which captures graded membership without the imposition of artificial or "crisp" boundaries.

Leading-limb laterality during locomotor initiation remains relatively unexplored in mammals with semi-arboreal locomotion. This study investigated forelimb preference during the transition from rest to locomotion in captive Chinese red pandas (Ailurus styani), a species naturally characterized by semi-arboreal locomotor adaptations. Behavioural observations of 35 individuals revealed no consistent individual level laterality after correction for multiple comparisons, and no population-level laterality was detected. A subtle negative association was found between age and Laterality Index (LL), indicating a slight trend toward increased left-limb use with maturation, while laterality strength showed no age-related change. No significant sex differences were detected in either the direction or the strength of laterality. For comparative context, reanalysis of published data from five captive giant pandas similarly revealed no significant locomotor laterality at individual or population levels. The absence of consistent laterality in both species may reflect adaptive motor strategies associated with semi-arboreal locomotion, where balanced bilateral coordination could enhance stability during movement across diverse arboreal and terrestrial substrates. This study provides the first investigation of locomotor laterality in Chinese red pandas and offers preliminary insights into how locomotor demands may influence motor organization in mammals with complex locomotor requirements.

A left-cheek bias has often been reported in previous studies of general portraiture. However, it remains unclear whether a similar tendency appears in offender portraits. The present study examined face orientation in newspaper portraits of murderers and fraud offenders. The analyses compared left-cheek and right-cheek presentations across male and female offenders in the two crime categories. No significant overall left-cheek bias was found. Face orientation did, however, differ significantly across the groups examined. Male murderers more often showed the right cheek, whereas female murderers, male fraud offenders, and female fraud offenders more often showed the left cheek. These findings suggest that offender portraits do not follow a single overall pattern of face orientation. Rather, face orientation in offender portraits differed across offender categories defined by crime type and gender.

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.