Many researchers have been interested in whether or not an individual’s birth order has an effect on intelligence. One of the first studies was carried out in the Netherlands during the early 1970s. 41 The test was called the “Raven,” which is similar to the I.Q. test. The researchers found a strong relationship between the birth order of the test takers and their scores on the Raven test. 42
In 1975, Zajonc and Markus developed the confluence theory to explain the negative effect of birth order on intelligence involving the data from the Dutch. 43 However, Rutherford and Sewell in 1991 tested the theory and found no support for it. They concluded that birth order effects did not exist.
44 On one side there are Zajonc and Markus, who state that birth order effects may be explained solely by family size and the spacing of births. With short birth intervals, increasing order of birth will be associated with lower intelligence levels. But with long birth intervals, this pattern may be reversed. 45 They show that the confluence model does not explain any relationship between birth order and intelligence that may exist in the American data. Up to date, there is no agreement between these opposing views. And such a debate may continue for years to come.
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