My colleague Lauren Brent and I have contributed a hypothesis & theory paper entitled "On the evolution of the serotonin transporter linked polymorphic region (5-HTTLPR) in primates" to a research topic in the open access journal Frontiers in Human Neuroscience. The paper has been accepted and should be online soon. Here is the abstract:
Some allelic variants of the serotonin transporter linked polymorphic region (5-HTTLPR) result in lower levels of expression of the serotonin transporter gene (SLC6A4). These low-expressing (LE) alleles are associated with mental-health disorders in a minority of humans that carry them. Humans are not the only primates that exhibit this polymorphism; other species, including some monkeys and apes, also have LE and high-expressing (HE) variants of 5-HTTLPR. We propose a behavioral genetic framework to explain the adaptive evolution of this polymorphism in primates, including humans. We hypothesize that both LE and HE alleles are maintained by balancing selection in species characterized by fluctuating levels of within-group competition. More specifically, we propose that LE carriers benefit from their hypervigilant tendencies during periods of elevated social competition, whereas HE homozygotes cope best when competition levels do not deviate from the norm. Thus, both alleles have long-term benefits when competition levels fluctuate substantially over time within a social group. We describe this hypothesis in detail and outline a series of predictions to test it. Some of these predictions are supported by findings in the current literature, while others remain areas of future research.UPDATE (11/8/13): Our article is now online and freely available here.