Population modeling aids in the understanding of factors that affect changes in population dynamics in any environment. Our study used population models and life-table response experiment analyses to explore the demographic mechanisms responsible for differences in population growth among zoological facilities and generations of captive squirrel monkey (Saimiri sciureus) populations. Variation in life history traits occurred among sexes, zoological facilities, and age groups. Those traits that vary the most include age at maturity, age at last reproduction, and fertility. Using prospective analyses, juvenile and adult survivals were predicted to be demographic traits with the greatest effect on population growth. However, differences may occur between what demographic traits are expected to cause changes in population growth rate and what traits actually contribute is important for management and conservation. Using life-table response (LTRE) analyses, the differences in population growth rate between zoological facilities and among generations of captive squirrel monkeys were almost entirely due to the contributions of fertility. For the generations of squirrel monkeys, LTRE modeling indicated that changes due to decreases in fertility that have developed over generations. Continuation of this trend would lead to declines in these captive populations, threatening a haven for this tropical primate species.