A burning topic in those days, as it remains today, was genotype × environment interaction (G × E). Although still unpublished at that point, John (Jinks) and David had shared a pre-print of their seminal 1970 paper on the application of Biometrical Genetics to human behavior (Jinks and Fulker 1970). Among other significant issues they addressed was that of G × E and, in particular, the possibility of examining the regression of absolute intrapair differences for monozygotic (MZ) twins on pair means as a key to characterizing the relationship between sensitivity to random environmental influences (intrapair differences in MZ twins) and average genetic liability (measured by pair means).
