Relationship between ordinal position and mental disorders

This study was concerned with the relationship of ordinal position to mental illness. It was limited to subjects from two-child families. A group of subjects who were only children was used for comparative purposes. The sample consisted of 614 subjects from two-child families and 245 only-child subjects. The sample was limited to white, Anglo-Americans who had been reared in an intact family, who themselves were older than 10 and had no sibling younger than 10, and who had functional mental disorders. Information on age, sex, sibling position, sex of sibling, age-spacing, psychiatric diagnosis, marital status, I. Q., educational attainment, maternal age at subject's birth, and socioeconomic class of the subject and his family of origin was obtained from psychological records. Simple and three-way chi squares were computed on the data in this study. The major findings of this study were: 1. Birth order was significantly related to the incidence of mental disorders in the case of the two-child family, and there were significantly more firstborn than secondborn subjects. 2. When factors such as sex, social class, and age were controlled, the effect of birth order on the incidence of mental disorders was no longer significant. 3. In several instances, birth order interacted with such factors. Firstborn males in social class I significantly outnumbered secondborn males; firstborn females significantly differed from secondborn females in the age range of 31-40; firstborn females in the less than two year agespacing between sibs significantly outnumbered secondborn females. 4. With respect to differential diagnosis, the effect of birth order was significant only among females when age-spacing was greater than four years. Firstborns were overrepresented in the schizophrenic category while secondborns were most numerous in the transient situational personality disorders. 5. Firstborns had significantly higher I. Q.'s than secondborns. 6. There was no significant difference in educational attainment on the basis of birth order. 7. Firstborn women were overrepresented in the multimarried/divorced category. 8. Comparison of only, firstborn, and secondborn subjects revealed the only significant diagnostic difference was between secondborn and only-child males. Analysis showed only-child subjects predominated in the schizophrenic category while secondborns were more often transient situational personality disorders. 9. Supplementary findings of this study were that psychiatric diagnosis was significantly related to sex of the subjects in all groups and remained significant when age and social class were controlled. Females predominated in the psychoneuroses while males tended to be diagnosed as personality disorders. Subjects 35 and younger tended to be diagnosed as personality disorders and schizophrenics while those over 35 were more often psyconeurotics. 10. I. Q. was significantly correlated with educational attainment and social class. Educational attainment was found to be significantly related to social class and maternal age at subject's birth while marital status correlated with psychiatric diagnosis and marital age.