Birth Order/Family Size & Intelligence Debate

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Birth Order/Family Size Intelligence Debate

• Keri-Ann Ritcey
• November 17, 2005

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What is this debate?

• Does ones birth order and family size have an
  effect on their intelligence?

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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)

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Approaches to Studying this

• Between Family Studies (cross-sectional data)
• Within Family Studies

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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).

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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)

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Three Main Models

• Resource Dilution Hypothesis
• Confluence Model
• Admixture Hypothesis

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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)

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Resource Dilution Contd
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Resource Dilution Contd

• 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 childs likelihood of success
• Surplus resources tend to be depleted when the
  number of children in the family increases
• (Downey, 2001)

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Resource Dilution contd

• 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 8th graders and found a negative
  relationship between the number of children in a
  family and the amount of resources available.

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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)

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Confluence model contd

• 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 11-2
  years and then predicts a positive influence for
  older children
• Why? -gt the benefits of teaching appear to grow
  less rapidly than the disadvantages of increasing
  the family size
• (Zajonc, 2001).

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Confluence model contd

• 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).

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Confluence Model Contd

• 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)

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Admixture Hypothesis

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

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The Admixture Hypothesis Contd

• 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)

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The Admixture Hypothesis Contd

• 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)

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The Admixture Hypothesis Contd

• 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.
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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).

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Niche Hypothesis contd

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

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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).

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Graduate Research

• Delroy Paulhus -The University of British
  Columbia (dpaulhus_at_psych.ubc.ca)
• Douglas Downey- The Ohio State University-Sociolog
  y Department (downey.32_at_osu.edu)
• Joseph Lee Rogers- University of Oklahoma
  (jrogers_at_ou.edu)
• Robert Zajonc- Stanford University
• (RobertZajonc_at_stanford.edu)

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Discussion

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

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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, 500- 612.

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References Contd

• 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).