The mating system of the golden lion tamarin is largely monogamous. When there are two adult males in a group only one of them will mate with the female. There are cases of a male mating with two females, usually a mother and daughter. Reproduction is seasonal and depends on rainfall. Mating is at its highest at the end of the rainy season between late March to mid-June and births peak during the September–February rains.
Females are $exually mature between of 15–20 months but it isn't until they are 30 months old when they can reproduce. Only dominant females can reproduce and will suppress the reproduction of the other females in the group. Males may reach puberty by 28 months. Tamarins have a four month gestation period. Golden lion tamarin groups exhibit cooperative rearing of the infants. It is due to the fact that tamarins commonly give birth to twins and to a lesser extent, triplets and quadruplets.
A mother is not able to provide for her litter and needs the help of the other members of the group. The younger members of the groups may lose breeding opportunities but they gain parental experience in helping rear their younger siblings. In their first 4 weeks, the infants are completely dependent on their mother for nursing and carrying. By week five, the infant spend less time on their mother’s back and begin to explore their surroundings. Young reach their juvenile stage at 17 weeks and will socialize other group members. The subadult phase is reached at 14 months and a tamarin first displays adult behaviors