Final answer:
Old World monkeys, found in Africa and Asia, often exhibit sexual dimorphism, while New World monkeys from the Americas do not show significant size differences between males and females.
Step-by-step explanation:
Old World monkeys, which include monkeys from Africa and Asia, are often sexually dimorphic, meaning there are significant size differences between males and females. This sexual dimorphism is believed to have evolved when one male had to defend multiple females. In contrast, New World monkeys from the Americas, such as tamarins and marmosets, do not show significant size differences between the sexes and tend to form pair bonds with their mating partners.