Various hypotheses derived from Annett's (1972; 1985) genetic theory of handedness are experimentally tested. Results from the first investigation show that excessive bias in favour of right handedness is due to a weakness in left rather than superiority of right hand skill, and is associated with poor nonverbal reasoning ability. A second investigation indicated that risks to reading problems were increased in children with either too little or too much bias in favour of dextrality. A further three studies investigated patterns of ability and disability at both laterality extremes. It was found that language deficits were more frequent in children reduced in bias towards the right hand. An attempt to find a task which those at the dextral tail of the laterality distribution were worse at than those at the sinistral tail met with inconclusive results. The experimental findings are, in general compatible with Annett's hypothesis of a human balanced polymorphism with heterozygote advantage for ability.