Caspi and coworkers reported a study of the SLC6A4 promoter insertion-deletion in the Dunedin Multidisciplinary Health and Development Study cohort.
They argued that the weakness of the association might hide larger effects in different environments, and cite animal models.
For instance, in rhesus monkeys, the short allele predicts low CSF serotonin metabolite levels (5-HIAA), but in deprived (peer-reared) and not in maternally reared monkeys [Bennett et al 2002]. A similar interaction is seen for ACTH response to stress [Barr et al 2004].