The Effect of Birth Order on Intelligence

The effect of birth order on intelligence was studied in an experiment. There are various theories involving this topic, it is somewhat controversial. On one side, the confluence model, proposed by Zajonc (1993) which attempts to explain the negative effect of birth order on intelligence. On another side, are those who believe that the confluence model does not hold up under careful scrutiny and may even be a theory that attempts to explain a social phenomenon that does not exist (Retherford & Sewell, 1991).

Students who were enrolled in Intermediate Psychology at Missouri Western State College were given a questionnaire composed of basic demographical information. The students were asked to give their birth order. Each students’ final grade in the class was used to measure intelligence along with the students’ overall grade point average. The statistics showed that the correlations were almost significant, but no real conclusions can be drawn from the information obtained. One interesting point to mention was that the first born children had a higher final grade in the class, while the last born children had a better grade point average.

There have been many researchers 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 1970’s. An intelligence test was administered to over 350,000 Dutch males when they turned 19 years of age. The test was called the “Raven” which is similar to the I.Q. test. The researchers (Belmont and Marolla, 1973) found a strong relationship in their data between the birth order of the men and their scores on the Raven test. Scores decreased as the family size increased and also with birth order. The confluence theory (Zajonc and Markus, 1975) was developed to explain the negative effect of birth order on intelligence involving the data from the Dutch. Since then, the theory has been elaborated to explain positive effects and nonlinear relationships between birth order and intelligence. Rutherford and Sewell (1991) tested the mathematical form of the theory using aggregate data, between-family data, and within-family data from the Wisconsin Longitudinal Study and found no support for the theory. Rutherford and Sewell (1991) also concluded that birth order effects may be “a social phenomenon that does not exist.”

There are definitely two sides to be debated about this issue. On one side there is Zajonc and Markus (1975) who state that birth order effects found in a set of data are hypothesized to be artifactual in that they 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 and with long birth intervals this pattern may be reversed. The other side being debated by Rutherford and Sewell (1957) uses the Wisconsin Longitudinal Study which was based on a random sampling of more than 10,000 Wisconsin high school graduates. The graduates were followed up in 1964 and 1975, with a 90 percent response rate. Data gathered concluded that there may be a relationship between birth order and measured intelligence in the Dutch data, but, the confluence model does not account for the relationship. The confluence model does not explain any relationship between birth order and intelligence that may exist in the Wisconsin data.