Attempts to explain behavior genetically face two major problems: the application of the concept of genetic coding and the theoretical possibility of decomposing behavior. This paper argues that using the notion of genetic coding is appropriate in explanations of protein synthesis but inadequate and even misleading in the context of explanations of behavior. Genes should be regarded as disparate components of mechanisms that account for behavior rather than as codes for behavioral phenotypes. Such mechanistic explanations, however, presuppose the possibility of decomposing behavioral phenotypes, which is strongly disputed by researchers holding an interactionist view of behavior. It is argued that these researchers fail to distinguish etiological from constitutive decomposition, and that their objections apply to the former but not to the latter kind. Constitutive decomposition might identify genes as disparate components and open up the possibility of explaining behavior mechanistically by isolating causal paths from genes to behavior. Finally, research on the single gene disorder phenylketonuria is introduced to illustrate and test these views. With respect to this disorder it is demonstrated that applying the concept of genetic coding would be inappropriate and misguiding, while nonetheless the phenotype is decomposable and can be explained mechanistically by singling out a genetic causal path.
