Published online by Cambridge University Press: 01 August 2014
It is immediately apparent, that the difficulties encountered in studies of the genetics of the mind are formidable, for several reasons. In the first place, the traits concerned are difficult or impossible to classify or to measure with accuracy. In the second place, they may be unstable towards environmental influence; and they vary with age, or exhibit periodic changes.
It is therefore not surprising that our understanding of genetic mechanisms is limited in those conditions where analysis has been carried out merely on the basis of the mental traits.
The situation is strikingly different in the conditions where an association has been found between a mental trait and some clearly recognisable physical or biochemical trait. As we all know, in several such cases an exact genetic analysis in Mendelian terms has been found possible, for instance in the amaurotic idiocies and phenylketonuria.