The variation observed in individual differences for normal and psychopathological behaviors has genetic factors as a major contributor at the most distal end of a complex gene-to-behavior pathway. Research into the etiologies of such major mental diseases as schizophrenia is facilitated by adopting the approach used for complex adaptive systems as pursued by those who study coronary artery disease and diabetes. Putative risk factors for developing the liabilities to the major disorders can be inferred from population genetic strategies using twins, families, and adoptees. Weights to indicate the relative importance of such risk factors require a perspective from the use of effect sizes and odds ratios so as to make the most efficient use of scarce resources. The challenge to the field of psychology is to join in with cross-disciplinary ventures and to adapt to rapid changes with innovations in research strategies.