The influences of family size and birth order on cognitive development in children are analysed. For this sample of low socio-economic status families, the abovementioned factors are negatively correlated to cognitive development, more markedly for large families than for one-child ones; family size effect is greater than that of birth order. The latter in turn depends on sibs' gender configuration and on parents' cognitive and personal characteristics. Mother's and father's influence on children's cognitive development characteristics are not equivalent.