A common group of marmosets is made up of 15 members, but usually has nine members. Common marmosets have a complex mating system. In a group of marmosets, there will usually be one or two females that will have babies and one male to be the father. The females are usually closely related, while the male is distantly related. If the male dies, the group will break up and form new groups. When marmosets get older, they leave the group they were raised with. It is not known why.