The Effect of Birth Order on Intelligence

Shelly Lowery
Missouri Western State College
December 5, 1995

Abstract

The effect of birth order on intelligence was studied in this 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.

Birth Order and Intelligence: Can You Relate?

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.

In my study I am attempting to show that there is an effect on intelligence by an individual's birth order. Possibly, if any significant statistics can be discovered I can further my studies in order to weed out any confounding variables. I would like to determine that there is differences in intelligence due to your birth order and that it is not just a social phenomenon by comparing first born to last born children and their final grades as measurment of intelligence.

Methods

Subjects

The participants used for this study were students at Missouri Western State College. About 150 students who were enrolled and w completed Intermediate Psychology were given a survey of birth order which included basic demographics which were completed by the students. Intelligence was measured by the students' final grades as determined by their professor. The surveys were given out in class on the first day and returned before the end of the class period.

Materials

A survey which included demographical questions were used during this study. I performed a one-way ANOVA to determine if the first born was different from the other siblings in the area of intelligence. I used MYSTAT in order to determine the standard deviation and correlation of intelligence and birth order. The students' final grades served as a measurement for intelligence.

Procedure

A questionnaire containing birth order questions and basic demographic statistics was given to students enrolled in Intermediate Psychology at Missouri Western State College. They completed the questionnaire and were instructed to return it. Each student was given an identification number to pair with their final grade in the class. The students were grouped into categories of first born, second born, third born, fourth born, and so on. The students were then ranked by grades along with their birth orders. I tried to determine the effect of birth order on intelligence. The students' final grade was determined by the professor.

Results

Previous research has demonstrated that the earlier-born children tend to score higher on tests of intelligence and aptitude than those born into the family later. The data compiled by my experiment shows almost significant results with the interactions between birth order and intelligence. An independent t-test found no real significance [t(150)= 1.93, p= .055] between the birth order and the final grade in the class. There was also no significant trend [t(150)= -1.42, p< .05]that birth order had any effect on grade point average. When comparing the average number of first born children in my study to the overall population, the percentage of students who were first born was 38.0% while the overall population of first born from 1970 to 1992 was 20.8% (National Center for Health Statistics, 1995). This study basically shows that there is no significance between birth order and intelligence and that the sample for the experiment is not representative of the entire population.

Discussion

The intent of this experiment was to show that birth order does have an effect on intelligence, that the first born children score better on intelligence tests. My findings indicate that birth order has no real effect on intelligence, whether you are first born or last born, it really doesn't matter.

This could be a study to investigate further with a larger sample size and with a sample that is more representative of the population as a whole. The participants were taken from a medium size, midwestern city which does not reflect well the intelligence level of a larger sized city, such as New York or Las Vegas. After completing the experiment I am inclined to take the side of Retherford and Sewell (1991) who believe that the difference in intelligence is due to a social phenomenon rather than birth order.

References

Bargh, J.A., Berbaum, M.L., Markus, G.B., Moreland, R.L., & Zajonc, R.B. (1991). One justified criticism plus three flawed analyses equals two unwarranted conclusions: A reply to Retherford and Sewell. American Sociological Review, 56, 159-165.
Hock, R.R. (1992). Forty studies that changed psychology. New Jersey: Prentice Hall.
National Center of Health Statistics. (1995). Vital statistics of the United States (DHHS Publication No. P25-1104). Washington, DC: U.S. Government Printing Office.
Retherford, R.D., & Sewell, W.H. (1991). Birth order and intelligence: Further tests of the confluence model. American Sociological Review, 56(2), 141-158.

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