The future of genetics belongs to molecular genetics in part because quantitative genetic research has made the case that genetic influences are important for most behavioural disorders and dimensions. Molecular genetics is now able to identify some of the specific DNA variants responsible for this heritability, and nothing will advance the field more than this (see below). Nonetheless, quantitative genetics such as twin studies will continue to play an important role in genetic research because it gives us the ‘bottom line’ of genetic influence regardless of how many genetic variants affect behavioural differences or how small and complex their effects might be. Much remains to be learned even about this rudimentary question of how much genetics affects many disorders (Plomin, DeFries, McClearn, & McGuffin, 2008). However, the greatest impact of quantitative genetics will come from research that goes beyond this basic question to investigate how genes have their effect. One example is the analysis of developmental change and continuity: Genetic differences between people account for most stability from age to age whereas environmental factors account for change. If most genetic action is involved in age-to-age stability, longitudinally stable phenotypes will be useful targets for molecular genetic research.
