SignificanceNew World monkeys (NWMs) are characterized by an extensive size range, and clawed NWMs (callitrichines) such as marmosets manifest diminutive size and unique reproductive adaptations such as twinning. Evolutionary explanations have been proposed for these traits, and with the common marmoset genome assembly the genetic underpinnings of these traits can now be explored. Callitrichine-specific nonsynonymous substitutions were identified inGDF9,BMP15, BMP4, andWFIKKN1. We postulate that positive selection affected NWM growth patterns, with callitrichine miniaturization coevolving with a series of reproductive adaptations that bear benefit when gestating multiples. Given the high rate of morbidity and mortality with human twins, future studies into callitrichine genomic adaptations will undoubtedly lead to unique insights of benefit to their human counterparts.