Laterality

How Many Handednesses Are There?

For more than a century, handedness has stood as the canonical example of individual difference in functional asymmetry. We routinely classify people as right-handed or left-handed, perhaps allowing a small space for the ambidextrous, or carving out a variable mixed-handed group in-between. This categorical view has become so deeply naturalised that it is often taken for granted. Yet accumulating empirical evidence challenges such binary understandings of laterality, prompting a deceptively simple but profound question: How many handednesses are there?

Empirically, the dichotomy for handedness in humans has always been leaky. Behavioural assessments reveal a continuous distribution of hand preference and manual performance. Individuals who write with the right hand may throw with the left; skilled musicians or athletes often display task-specific lateralisation patterns. The so-called mixed-handers or inconsistently handed individuals blur the boundaries between the two dominant categories, while population-level variability resists a neat classification. Even among researchers, operational definitions of this middle group diverge considerably. Comparable complexity is evident in non-human species, where limb preferences vary in strength and direction according to species, task, and development stage.

At the neural level, it may be even more complex. Functional cerebral asymmetries in the motor cortex, cerebellum, and corpus callosum vary across individuals and tasks, and show only partial, or sometimes negligible, correspondence with self-reported hand preference or hand-performance differences in humans.

Genetic findings further complicate the story. Rather than identifying a single handedness gene, evidence supports a polygenic inheritance with modest explanatory power, consistent with probabilistic mechanisms that interact with environmental and developmental factors. Early preferences can shift in direction or strength through experience, training, or sociocultural pressures against left-hand use, underscoring that laterality might be better understood as a dynamic process rather than a fixed trait.

Historically, however, laterality research in humans has been shaped by binary thinking. Research of motor control, language lateralisation, and other forms of functional cerebral asymmetries often restricted participation to right-handers, treating left-handers as anomalies to be excluded rather than as valuable sources of variation. Whether motivated by methodological convenience or implicit bias, this exclusion has constrained our understanding of the full spectrum of human lateralisation. Encouragingly, this practice is now changing. Increasing numbers of studies now include participants across the entire range of hand-preference scores, reflecting a growing recognition that individual variability is central to understanding asymmetry.

Binary concepts are, in many ways, a relic of an earlier scientific era. The growing recognition of continuum models across psychology and many other related disciplines has revealed a world of nuance and complexity in understanding human and non-human behaviour and mental processes. No longer can we neatly divide people into discrete categories or label experiences as wholly normal or abnormal, introvert or extrovert, male or female. Contemporary science increasingly acknowledges the fluidity and gradation of traits, preferences, and mental states, dimensions that often lie along spectra rather than within rigid boundaries.

In that spirit, just as contemporary debates have expanded our understanding of sex and gender beyond binary frameworks, we might ask whether the right–left dichotomy is an oversimplification of a multidimensional space of handedness and manual asymmetries. Handedness may not be a single construct, but rather a cluster of related yet partially independent traits, including preference, performance, consistency, and neural organisation.

This conceptual pluralism invites an epistemic shift. If we accept that there may be many handednesses, how should we study them? Should classification systems reflect continua rather than categories? Or might existing typologies still hold pragmatic value? Should we privilege subjective preference, behavioural proficiency, neural lateralisation, or genotypic disposition? What could a non-binary model of handedness contribute to theories of hemispheric specialisation, motor control, and embodied cognition?

Through the special issue How many handednesses are there?, we do not aim to replace one taxonomy with another, but to open a dialogue across disciplines and species. Laterality invites short commentaries (5-10 pages) addressing empirical, theoretical, methodological, and philosophical implications of expanding the concept of handedness. Submissions may explore behavioural or neurobiological evidence, developmental or cultural perspectives, interspecies comparisons, or reflections on measurement and definition.