The Role Of Learning

There seems to be little doubt that the relative frequency of left-handedness (e.g., Tambs et al., 1987; Gilbert & Wysocki, 1992; Davis & Annett, 1994; Hugdahl et al., 1996) and left-footedness (Porac, 1996; Bell & Gabbard, 2000) declines with age and now is less than it was a century ago. The reason for this is much debated, but one theory is that it represents a relaxation in recent years of cultural pressure toward obligatory use of the right hand for certain tasks (Brackenridge, 1981). Implicit in this view is the notion that handedness is modifiable through cultural or environmental factors. If that is so, might not such factors explain handedness in the first place?

The idea that handedness is a learned characteristic has a long history. Some authors, though admitting a congenital bias toward right-handedness in some or all children, have concluded that learning is the major determinant (e.g., Humphry, 1861; Perelle et al., 1981; Provins, 1997a, 1997b), while some have strongly opposed this view (Buchanan, 1862; Annett, 1985; McManus & Bryden, 1993). Other writers have been more cautious. Heilman, Coyle et al. (1973), for example, note that ''although dexterity is normally greater in the dominant hand, one must keep in mind the possibility that dexterity is the result of preferred use, rather than vice versa'' (p. 25).

Provins (1997b) reiterated his belief that handedness derives from practice effects. Briefly, he contends that ''What is genetically determined is a neural substrate that has significantly increased its functional plasticity in the course of evolution____What is fine-tuned is the relative motor proficiency or skills achieved by the two sides in any given task according to their use and the demands made on them as a result of social pressure, other environmental influences or habit'' (p. 556).

An alternative opinion was expressed by McManus et al., (1986), who argued that practice is unlikely to be the cause of performance differences between the hands, since degree of improvement in a simple tapping task was similar for both preferred and nonpreferred hands (and for typists and piano players as well as for nonspecialist participants).

The view that handedness is learned leaves unanswered the question of why the cultural bias should always lead to the majority being right-handed. Nor does this argument explain why approximately 80% of left-handers have two right-handed parents, or why two left-handed parents do not have a majority of left-handed offspring (Annett, 1983; McKeever, 2000). More generally, it is difficult to see how genetic models can predict family handedness data so successfully if learning is the major cause of hand differences.

A further objection to a purely learning-based account of handedness is that it cannot easily explain those findings on fetal thumb-sucking (Hepper et al., 1991) or arm movements (Hepper et al., 1998) which suggest that some manual functions are lateralized prior to birth. (Of course it remains to be shown that these early manifestations of asymmetry relate to adult handedness).

Finally (but not exhaustively), handedness has been related (frequently if not convincingly) not only to physical features connected with, for example, the nose, (Sutton, 1963), testicles (Chang et al., 1960), and handprints and fingerprints (Newman, 1934), but also to behavioral phenomena such as eye dominance (McManus et al., 1999; Annett, 1999b, 2000) and laterality of jaw movements (Koch, 1933), which are difficult to reconcile with an effect of learning. The fact that anatomical brain asymmetry has been related to a familial as well as a personal history of left-handedness (Steinmetz et al., 1991) is particularly problematic for a learning account.

No one would deny that a change of handedness may be brought about by early cerebral damage (see Satz, 1972) or unilateral upper limb injury (Dellatolas et al., 1993). However, it is also true that individuals appear to differ considerably in the extent to which their nervous system "resists" attempts at retraining after loss of one hand (Smith, 1927, p. 189) or temporary injury (Porac, 1995; see also Beaton et al., 1994). It is also undeniable that parts of the brain concerned with manual dexterity can undergo considerable functional and even structural modification through extensive training (e.g., Wang et al., 1995; Elbert et al., 1995). However, it is difficult to see on a purely learning-based account why performance levels of the preferred hand should be approximately constant across different subgroups while that of the nonpreferred hand varies. It is also difficult to accept that the sex difference in handedness is entirely attributable to learning, though it may contribute to some extent (see Porac et al., 1986).

In view of all the arguments against a learning theory of hand preference and skill, it seems reasonable to conclude that biology plays the major role but that learning may play some part in helping to "fix" the magnitude of handedness within a given direction, left or right.

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