Most cases have found Leontopithecus chrysopygus to be monogamous when mating but there has been some polyandy observed in certain populations. In populations where there is more than one adult male per social group the female may mate with several males to confuse the males as to paternity of her young. This causes males to provide assistance in the care of offspring that may be their own.
Golden-rumped lion tamarins give birth most frequently to twins, though triplets and quadruplets have been reported. In other species of lion tamarin, gestation lasts from 125 to 132 days. Lion tamarins give birth during the rainy season, usually from September through March.
Both male and female golden-rumped lion tamarins aid in the rearing of offspring. The young are born well-furred and with their eyes open, but are entirely dependent on adults for their care. For the first 2 to 3 weeks newborns stay primarily with the mother. After three weeks the father will carry the young for much of the day, bringing them to their mother every 2 to 3 hours for feeding. The offspring are weaned after 2 to 3 months but usually don't leave the family group until they reach sexual maturity, at 16 to 24 months. (Nowak 1999)