In a January 2014 issue of PNAS Harris et al. analyze the common marmoset genome to explore the genetic basis of the species’ unique reproductive adaptations, including multiple gestations (i.e. dichorionic twins and trichorionic triplets). The authors identified callitrichine-specific nonsynonymous substitutions that they found to be associated with the inhibition of growth factors and bone morphogenetic proteins. They postulated that positive selection affected New World monkey growth patterns, with callitrichine miniaturization coevolving with reproductive adaptations that bear benefit when gestating multiples. Polymorphisms in GDF9 and BMP15 were candidate genes for twinning in callitrichines based on studies in other species, and two other candidates were identified, BMP4 and WFIKKN1. These analyses contribute a set of candidate loci to be further characterized in human populations with a higher heritability for twinning.