Failure to support the right‐shift theory s hypothesis of a ‘heterozygote advantage’ for cognitive abilities

Abstract

Annett's (1985) 'right-shift' theory of language dominance and handedness posits three genotypes, rs++, rs(+)- and rs(-)-, and Annett has hypothesized that there are cognitive ability correlates of these genotypes. The rs++ genotype person is held to be 'at risk' for maldevelopment of spatial or other right hemisphere-based cognitive abilities, and the rs(-)- genotype individual is held to be at risk for maldevelopment of phonological abilities. Noting that there must be some adaptive advantage conferred by the heterozygous genotype for it to have survived over a presumably long period of evolution, Annett has hypothesized that heterozygotes are afforded an adaptive advantage over homozygotes because of their freedom from 'risks' to intelligence generally. Annett and colleagues have used two different indices, or markers, from which they have inferred differing concentrations of the three genotypes within groups of participants. One marker, based on responses to hand preference items of the Annett Handedness Inventory, was found by Annett (1992) to support her theory in that the least dextral of right-handed participants did best on spatial tests. The other marker Annett has used is based on the degree of right-hand advantage on a simple peg moving speed task. The present study utilized both methods and studied the performances of 259 dextral college men and women on two tests of mental rotation ability and two tests of verbal abilities. Results were not supportive of the heterozygote advantage hypothesis, and suggested that visuospatial ability was modestly related to greater dextrality of participants.

Do left-handed children have advantages regarding school performance and leadership skills?

“…Some studies found that left-handers have a lower intellectual performance (Crow, Crow, Done, & Leask, 1998;Hicks & Beveridge, 1978;Lamm & Epstein, 1999;McManus & Mascie-Taylor, 1983;Miller, 1971;Olivier, 1978). However, some studies found no differences between right-and left-handers (Cerone & McKeever, 1999;Gignac & Vernon, 2004;Hardyck, Petrinovich, & Goldmann, 1976;Hicks, 1976;Nettle, 2003;Newcombe & Ratcliff, 1973;Wittenborn, 1946). Finally, some studies found an advantage for left-handers (Annett, 1970;Deutch, 1978;Ehrman & Perelle, 1983;Hicks & Dusek, 1980).…”

“…For example, the``right-shift theory'' states that handedness is a by-product of a factor that induces lateralised speech representation in the brain, and that there is over-dominance at this major genetic factor, RS+/± individuals having an overall cognitive advantage over either RS+/ + or RS±/±, due to interference of RS+ and RS± with distinct cognitive functions (Annett, 1985). However, this model has not received strong empirical support (Cerone & McKeever, 1999;McKeever, Cerone, & Chase-Carmichael, 2000). Actually, laterality is likely to have a more complex genetic determinism (Francks, Fisher, MacPhie, Richardson, Marlow, & Stein, 2002;Laval, Dann, Butler, Loftus, Rue, & Leask, 1998).…”

Some evidence of a female advantage in object location memory using ecologically valid stimuli

“…We estimated that the participants would have been drawn from a relatively homogeneous background, and consequently possess similar socioeconomic backgrounds and levels of intelligence. The impact of sinistrality upon spatial ability is an ongoing research issue (see Casey 1996;Cerone and McKeever 1999); thus the present sample was limited to dextral participants. From a short questionnaire, participants provided information considered relevant to the study (e.g., handedness, color blindness, gardening experience, specific knowledge of plants).…”

Ambidexterity and magical ideation

“…Annett has proposed that the RS+ allele may favour verbal capacities but heighten the risk of spatial deficits, whereas the 80 BARNETT AND CORBALLIS RS7 allele may enhance spatial and motor skills but increase the risk of verbal deficits. Although she and her colleagues have presented a number of findings consistent with this expectation (e.g., Annett, 1993Annett, , 1999Annett & Kilshaw, 1982;Annett & Manning, 1990), a number of other studies have produced evidence inconsistent with it (e.g., Cerone & McKeever, 1999;McManus, Shergill, & Bryden, 1993;Palmer & Corballis, 1996;Resch et al, 1997).…”