Commentary: Variation and Causation in the

Science is about causes, period. For too many years, mainstream behavioural genetics was based on a watery version of classical population genetics that entailed partitioning the variation of a trait into three (or occasionally a few more) components. I pre-sume the basics of the process are familiar to readers of this comment. Using pairs of twins or other vari-ation in genetic relatedness, variation in a trait can be partitioned into a component (denoted A) ‘attribut-able to ’ (the vagueness of this half statistical, half biological phrase protected the field from recognizing its limitations) the additive effects of genes; a com-ponent called C, for common environment, attribut-able to environmental effects making children raised in the same family similar to each other, and finally