Questions related to the problem of accepting a small reward immediately or waiting for a bigger reward were given to 180 fourth-grade children composed of three ethnic groups sampled from a relatively poor socioeconomic region of a South-Western area in the United States. In addition questions related to the factor of trust in the promises of investigators to deliver the bigger reward were undertaken. The sample consisted of 60 Anglos, Blacks, and Mexican-Americans, respectively; each ethnic group divided in turn into an equal number of boys and girls. Results showed that at the fourth-grade level Black and Mexican-American children were more prone than Anglo children to accept the immediate gratification rather than choose the later and bigger reward. No sex differences within each ethnic group were found, with the exception of the Mexican-American group for one out of the three conditions tested. The factor of mistrust in the promises of the investigators was noticeable in the Black children, despite the fact that they were tested by Black investigators.