1 Birth Order/Family Size & Intelligence Debate Keri-Ann Ritcey November 17, 2005

2 What is this debate? Does one’s birth order and family size have an effect on their intelligence?

3 Why is this debate of interest?? Everybody has a birth order We often refer to birth order in general conversation. Causal simplicity Parents can explain behavior without any responsibility or accountability Relatively stable overtime People tend to generalize what they observe within their own families to other families (Rogers, 2001)

4 Approaches to Studying this: Between Family Studies (cross- sectional data) Within Family Studies

5 Between Family Designs Advantages: Data can be obtained from a large number of people relatively quickly Disadvantages: May not contain any real within family variation Any observed differences may be coming from outside of the family (confounds) (Rogers, 2001).

6 Within Family Designs Advantages: eliminates between family confounds such as social class or parental personality Disadvantages: Within family designs may also have confounds Assumption that children within a family share the same family size at any given time (Michalski & Shakelford, 2001)

7 Three Main Models Resource Dilution Hypothesis Confluence Model Admixture Hypothesis

8 The Resource Dilution Theory Interested in how the family structure may advantage or disadvantage children Proposes that since parental resources are limited, the addition of children to a family will result in the dilution of resources available to each child (Downey, 2001)

9 Resource Dilution Cont’d

10 Resource Dilution Cont’d However, not all resources are equally important. Parental resources can be defined as either surplus or base resources Base resources are those required for basic survival Surplus resources are those believed to increase a child’s likelihood of success Surplus resources tend to be depleted when the number of children in the family increases (Downey, 2001)

11 Resource Dilution cont’d Evidence: Blake (1989) conducted a cross sectional study and found that as the number of children in a family increased, the number of years children spent in school decreased. Downey (1995) conducted a longitudinal study of 24,000 8 th graders and found a negative relationship between the number of children in a family and the amount of resources available.

12 Confluence Model The mental maturities of children growing up in the same families flow together over time in their influence on each other, changing constantly over time and changing most profoundly when new offspring join or leave the family. Predictions: As the number of siblings increases the overall intellectual environment of the family decreases. However, the teaching function lessens the effect of expanding the family. Only children or last born children are predicted to have lower scores because they have no one to teach. (Zajonc, 2001)

13 Confluence model cont’d What is the teaching function? Predicts that the effects of birth order and family size are age specific Predicts a negative influence or no influence of birth order for children less than age years and then predicts a positive influence for older children Why? -> the benefits of teaching appear to grow less rapidly than the disadvantages of increasing the family size (Zajonc, 2001).

14 Confluence model cont’d Based on the assumption that each child enters into a different family environment than the last and that each child changes and keep changing the family environment. Since this hypothesis predicts age specific effects, a giant confound is created when studied within families. Therefore, a longitudinal approach is required (Zajonc, 2001).

15 Confluence Model Cont’d Evidence: Zajonc (2001) used between family data to show that population trends vary with the age that children are tested. Using the Iowa School Proficiency test he plotted the average scores of 10 grades together with birth order. Results were consistent with the predictions of the confluence model. (Zajonc, 2001)

16 Admixture Hypothesis Propose that “the apparent relationship between birth order and intelligence has been a methodological illusion” (Rogers, 2000) Why?->Because the data from between family studies are filled with so many biases that they are virtually useless (Zajonc, 2001).

17 The Admixture Hypothesis Cont’d Evidence: When studied within family birth order effects disappear Between Family Studies falsely infer within family processes To further illustrate the issue of using between family data: Imagine comparing a large middle class white family in Port Williams to the second born child in a medium sized affluent Black family in Toronto to a third born child in a small low income Hispanic family in Montreal What possible confounds exist here? (Rogers, Cleveland, van den Orrd & Rowe, 2000)

18 Argue that the source of between family patterns must come from outside of the family Must control for between family differences Research community must be prepared for the real possibility that there is no relationship Inconsistent patterns likely exist within families (Roger et al., 2000) The Admixture Hypothesis Cont’d

19 The Admixture Hypothesis Cont’d Rogers et al. suggest that parental IQ may be an alternative explanation Mothers' Average AFQT IQ Percentile Scores by Family Size for the Total Sample of Mothers Note. AFQT = Armed Forces Qualifying Test.

20 Niche Hypothesis An alternative explanation: Since first borns have the first choice of niche, they may attempt to please their parents in traditional ways. When other children arrive first borns now have to deal with the potential threat to their hierarchy. As a result, first borns tend to be conscientious and conservative. In contrast, later borns may use alternative ways of distinguishing themselves. (Paulhus, Trapnell & Chen, 1999).

21 Niche Hypothesis cont’d Paulhus et al (1999) tested this hypothesis using a within family sample. How?->He asked siblings to rate themselves and their siblings on various personality and achievement dimensions. Findings-> First borns are more intellectually achieving. Caution-> sig. results may have been the results of stereotypes which impact the individuals perceive their siblings and their selves (Paulhus et al., 1999)

22 Suggestions for future research Create designs that can distinguish between family processes Possibility of pairing to help eliminate between family confounds Determining what alternative factors could be causing these patterns in Cross-sectional data Develop more precise research designs capable of detecting very small effect sizes (Rogers, 2001).

23 Graduate Research Delroy Paulhus -The University of British Columbia Douglas Downey- The Ohio State University-Sociology Department Joseph Lee Rogers- University of Oklahoma Robert Zajonc- Stanford University

24 Discussion How is this research relevant to society? What factors do these researchers fail to consider? How does this apply to your family?

25 References Blake, J. (1989). Family size and achievement. Los Angeles: University of California Press. Downey, D. B. (2001). Number of siblings and intellectual development: The resource dilution explanation. American Psychologist, 56 (6-7). Michalski, R. L., & Shakelford, T. K. (2001). Methodology, birth order, intelligence, and personality. American Psychologist, 56 (6-7). Paulhus, D. L., Trapnell, P. D., & Chen, D. (1999). Birth order effects on personality and achievement within families. Psychological Science, 10 (6). Rogers, J. L. (2001). What causes birth order-intelligence patterns? The admixture hypothesis revived. American Psychologist, 56 (6-7). Rogers, J. L., Cleveland, H. H., van den Oord, E., & Rowe, D. C. (2000). Resolving the debate over birth order, family size, and intelligence. American Psychologist, 55,

26 References Cont’d Zajonc, R. B. (2001). Birth order debate resolved? American Psychologist, 56 (6-7). Zajonc, R. B. (2001). The family dynamics of intellectual development. American Psychologist, 56 (6-7).