The field of psychiatric genetics has failed to find common gene variants of large effect in the development of depression. Many of the family, twin, and adoption studies supporting this argument have too small patient cohorts. No findings have been consistently replicated as well. Additionally, the effects of genetic variants identified such as 5-HTT, are very weak, with odds ratios of 1.0 to 1.2. The impact of lifestyle and environmental factors seem to be larger than genetic predisposition.
Risch et al. (2009) found in their meta-analysis that there is no correlation between 5-HTT variation and depression. Therefore, the reliability of Caspi et al. (2003) is very low.
Establishing such a concrete relationship between genes and depression as in the argument is very problematic. It may lead to genetic determinism –the attribution of the formation of traits to genes where genes are ascribed more causal power than what scientific consensus suggests. Researchers should emphasize that the relationship is only correlational and not causal.