Infant development is influenced by structural variables of the family environment (educational resources as father's level of education and economic resources as housing and father's activity) and by functional variables such as family relationships.

In a transversal randomised study, 400 children 6, 12, 24 and 60 months were evaluated. The socio-economic interview (SES) was used as an indicator of the structural variables, the HOME Inventory for the functional varibles and the Bayley II Scale and the WPPSI for the mental/motor development and intellectual coeficient. A direct and positive association was demonstrated between infant development and the HOME Inventory. At the same time, homes from higher socio-economic status obtained better scores in the HOME inventory. That is, children from better social levels have better structural conditions of life and higher level of richness in their environment as well as higher levels in developmental scores and IQ. Logistic and multiple regression show that when the SES effect is controlled, the greatest determinant over infant performance is related to functional variables of the environment and not to the structural ones.

In conclusion, family relationship-specially mother/father and son interactions are closely associated with infant development and are independent of SES. These findigns allow to speculate that programs directed to promote rich experiences for infants could increase their development and educability at school entrance, in spite of the structural limilations of their society.