Final answer:
Species such as tamarins and marmosets, which are small New World monkeys, live in small social groups with a high incidence of twins and share the responsibility for raising young.
Step-by-step explanation:
The species of primate that is small in stature, known for having a high occurrence of twins, living in small social groups, and collectively caring for young, is most likely referring to certain species of New World monkeys, such as tamarins or marmosets. These species are known for forming pair bonds and cooperatively raising their offspring, displaying less of a size difference between the sexes than is typical in other primate groups. Furthermore, the shared responsibility for caring for the young is not exclusive to the biological parents, showcasing an advanced level of social structure within their communities.