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Are apparent sex differences in mean general intelligence created by sample restriction and increased male variance?

Are apparent sex differences in mean general intelligence created by sample restriction and increased male variance?
Are apparent sex differences in mean general intelligence created by sample restriction and increased male variance?
This study investigated the possibility that apparent sex differences in IQ are at least partly created by the degree of sample restriction from the baseline population. We used a nationally representative sample, the 1970 British Cohort Study. Sample sizes varied from 6518 to 11,389 between data-collection sweeps. Principal components analysis of scores obtained on four cognitive tests administered at age 10 was used to obtain estimates that we name ‘IQ’. These age-10 scores were then used to estimate the sex differences at age 10, and also among participants in the two later waves, at age 26 and 30. At age 10, there was a small but significant advantage for boys (Cohen’s d = 0.081). Boys had greater variability in these IQ scores. We then investigated how this very small male advantage at 10 changed with sample restriction. We used the same IQs obtained at age 10, but considered only those subjects who returned for data-collection sweeps at ages 26 and 30 years. Subjects returning at age 26 and 30 were more likely to be females and to have higher age-10 IQ scores. Attrition at age 30 was 28% and the male advantage in IQ scores increased by 15%. Attrition at age 26 was 43% and the male advantage in IQ scores increased by 48%. The findings underline the importance of monitoring attrition in longitudinal studies, as well as emphasising the need for representative samples in studying sex differences in intelligence. A proportion of the apparent male advantage in general cognitive ability that has been reported by some researchers might be attributable to the combination of greater male variance in general cognitive ability and sample restriction, though this remains to be tested in a sample with an appropriate mental test battery.
42-47
Dykiert, Dominika
fddf5d63-822d-449b-9bcb-308e3a95d9f0
Gale, Catharine R.
5bb2abb3-7b53-42d6-8aa7-817e193140c8
Deary, Ian J.
027158ae-fbfb-40ea-98b1-32d2690499ac
Dykiert, Dominika
fddf5d63-822d-449b-9bcb-308e3a95d9f0
Gale, Catharine R.
5bb2abb3-7b53-42d6-8aa7-817e193140c8
Deary, Ian J.
027158ae-fbfb-40ea-98b1-32d2690499ac

Dykiert, Dominika, Gale, Catharine R. and Deary, Ian J. (2009) Are apparent sex differences in mean general intelligence created by sample restriction and increased male variance? Intelligence, 37 (1), 42-47. (doi:10.1016/j.intell.2008.06.002).

Record type: Article

Abstract

This study investigated the possibility that apparent sex differences in IQ are at least partly created by the degree of sample restriction from the baseline population. We used a nationally representative sample, the 1970 British Cohort Study. Sample sizes varied from 6518 to 11,389 between data-collection sweeps. Principal components analysis of scores obtained on four cognitive tests administered at age 10 was used to obtain estimates that we name ‘IQ’. These age-10 scores were then used to estimate the sex differences at age 10, and also among participants in the two later waves, at age 26 and 30. At age 10, there was a small but significant advantage for boys (Cohen’s d = 0.081). Boys had greater variability in these IQ scores. We then investigated how this very small male advantage at 10 changed with sample restriction. We used the same IQs obtained at age 10, but considered only those subjects who returned for data-collection sweeps at ages 26 and 30 years. Subjects returning at age 26 and 30 were more likely to be females and to have higher age-10 IQ scores. Attrition at age 30 was 28% and the male advantage in IQ scores increased by 15%. Attrition at age 26 was 43% and the male advantage in IQ scores increased by 48%. The findings underline the importance of monitoring attrition in longitudinal studies, as well as emphasising the need for representative samples in studying sex differences in intelligence. A proportion of the apparent male advantage in general cognitive ability that has been reported by some researchers might be attributable to the combination of greater male variance in general cognitive ability and sample restriction, though this remains to be tested in a sample with an appropriate mental test battery.

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Published date: January 2009

Identifiers

Local EPrints ID: 70955
URI: http://eprints.soton.ac.uk/id/eprint/70955
PURE UUID: e6301771-6e82-46b1-8403-c069434124f0
ORCID for Catharine R. Gale: ORCID iD orcid.org/0000-0002-3361-8638

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Date deposited: 05 Jan 2010
Last modified: 09 Nov 2021 02:49

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Contributors

Author: Dominika Dykiert
Author: Ian J. Deary

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