400,000 nineteen year old males. At this age every male was "required to appear for examination to determine their fitness for military induction". They tested IQ against family size and birth order and found that "children from larger familes tend to make poorer showings on intelligence tests and on educational measures, even when social class is controlled". They then went on to say that they found "Within each family size firstborns scored better on the tests than later born children". Furthermore, as birth order increased the average test scores decreased.
Frank Swulloway, an American psychologist, published his book, Born To Rebel: Birth Order, Family Dynamics and Creative Lives, on the 3rd of Decemeber 1998. In his book he stated that "Birth Order fosters differences in personality that in turn correlate with differences in creative achievement. Swulloway believed that Birth order had a "direct connection" to the 'Five Big Personality Traits'.
He conducted a study that involved 6053 individuals aged 8 to 95. "Subjects rated themselves on nine-step bipolar scales using adjective pairs that were chosen to represent... the five factor model". His reseults showed that "23 of the 30 adjective pairs [showed] significant differences and 26 of the 30... produced correlations in the expected direction". Many critics such as Toni Fablo and Fred Townsend argued against his theories extensively.
Many psychologists believe that there is a coherant correlation between birth order and personality, yet many others tend to disprove it. Studies related to the subject struggle to control the long list of variables that come with the subject. It is hard to understand fully whether there is actually a sizeable link between birth order and personality.