The popular view among American psychologists has been that there is a continuity of psychological structure that is shaped by early experience. Data gathered in studies of Guatemalan villages imply serious discontinuities in the development of particular cognitive competencies and capacities for affect through preadolescence. The first two years of life are not a good predictor of future functioning in all environmental contexts. Separate maturational factors seem to set the time of emergence of basic cognitive functions, although experience can slow or speed up that emergence to a degree. In the Guatemalan villages observed, infants are permitted very little activity in the first 15 months, are not allowed outside, have little to play with, and are seldom played with. These infants exhibit extreme motoric passivity, but they develop normally in childhood. Identical procedures involving color and movement were administered to American and Guatemalan infants. Guatemalan infants were significantly less attentive than the Americans, and Americans had longer fixation times. To test the reversibility of the apparent slowing of cognitive growth in the Guatemalan child, cognitive tests were administered to rural and urban middle-class and economically disadvantaged preadolescents. Their performance on tests of perceptual analysis, perceptual inference, recall and recognition memory was comparable to American middle-class norms. Certain data suggest that economically disadvantaged American and Guatemalan children aged 5-9 are from one to three years behind middle-class children in demonstrating some cognitive skills but that these competencies emerge by age 10 or 11. (KM)