 Prior to the first official scale by Nancy Bayley, research was conducted to determine which important variables should be included in a cumulative developmental test for infants.In 1965, Nancy Bayley conducted an experiment examining mental and motor test scores for infants aged 1 to 15 months, comparing sex, birth order, race, geographical location, and parental education.No differences in scores were found for either scale between boys and girls, first-born and later-born, education of either father or mother, or geographic residence.