Insight can be gained into the role of specified cultural variables in human development if care is taken in a study to include subcultural variations which can be matched cross-bulturally, to employ well-trained native examiners who have been calibrated cross-culturally, to use techniques which can be defended, and to involve the close and continual collaboration of investigators sensitive to the above issues. This paper reports findings from six years of repeated testing of children from Texas and Mexico, who were selected to represent a broad range of working-class, business, and professional families. A complex analysis-of-variance design was constructed with five main factors: 1) socioeconomic status; 2) sex; 3) age group when tested initially; and 5) year of repeated testing or trial. The main effects for culture proved highly significant, revealing important differences in developmental trends for the entire age span of 6-17 years in Mexican and American children. This study reports only highlights of a much larger research program involving hundreds of children and their families. Additional studies are aimed at gaining greater insight into the complex relationships between cognitive, perceptual and personality measures on the one hand and family life-style, home environment and school performance variables as these aspects of personality development change over time in the two cultures. (JMB)