Some investigators have reported findings suggesting that many traits may reflect at least to some extent a child’s position in his family. The evidence, though, is not always conclusive. On the average firstborn weighs less at birth than later children. Within two or three years, he is heavier than his siblings will be when they reach that age. But by adulthood there is no significant difference among them in weight and height.
Other studies indicate that asthma tends to strike firstborn more often than other children; duodenal ulcer, the lastborn; and that by age four later-born have twice as many infections as do firstborn. The findings about mental illness seem somewhat contradictory. Some studies indicate a slightly greater tendency toward schizophrenia among later-born children, especially girls. Others find that firstborn are referred more often to clinics because of anxiety and severe psychotic symptoms. There is some suggestion, too, that later born children are more likely to be mentally retarded.
Some psychiatric studies suggest that firstborn are more likely to be introverts and later-born to be extroverts, although there are many striking exceptions among celebrated men and women. Analyses of mother’s reports indicate that they find their first-born likely to be more timid, tense, and fearful than later children. At least, one psychological study indicates that firstborn tend to have more extreme fears related to social and interpersonal situations such as being alone and parting from friends.
Later-born seem more disposed toward temper tantrums, bedwetting, and nightmares and more likely to be more interactive, negative, destructive, and given to lying. At child guidance centers, firstborn, especially girls, are brought in more often with behavior problems, ranging from school phobia to stealing, but this could be a result of the experience and worries of young parents. It is in the area of intellectual achievement that firstborn, including children, seem to stand out. –Lawrence Galton.