The reproductive, biological and socio-economic characteristics of a sample of 4,952 subjects derived from a Peruvian population of low and medium socio-economic status were studied. The study suggests that under conditions of poverty there exists a symbiotic relationship whereby low socio-economic status is associated with a less efficient mechanism to control family size. This characteristic permits the mother of low socio-economic status to attain a more complete child-bearing period. As childhood mortality is inversely related to socio-economic status, so, with an increase in childhood deaths there is an increase in live births in a compensatory fashion. High fertility is the net result of these interactions. The implications of these findings to attempts to decrease population fertility through birth control alone are discussed
