The increase in one child families in the 1980's is analogous to the one child family rate during the Depression years. Although family size was limited for economic reasons during the Depression, current increases in the number of one child families are attributed to advances in contraceptive technology, increased employment of women, inhibiting economic factors, and increased marital instability. The increased numbers of voluntory one child families, may result in a child psychologically different from the only child of past generations. The well-known confluence model of intellectual development, which posits that intellectual development is determined by the level of family intelligence, is currently in doubt due to the recognition of various confounding factors, (e.g., father absence, socioeconomic status), and subsequent research. Studies of achievement with only children have shown that only children perform better academically than others. Parental expectation, financial abilities, and an uninterrupted relationship with the child are possible reasons for this high achievement. Self-esteem studies have placed only children above, below, and at par with others. In interpersonal relations only children have been found to be less affiliative than others, which may be the result of the relatively large amounts of affection they receive from their parents, rather than a lack of social skills. Only children appear to be mentally healthy, with many studies indicating that only children are underrepresented in psychiatric literature. Future research should focus on factors other than sibling absence in defining characteristics of the only child. (BL)