Intelligence and brain size
Does size matter?
Does brain size correlate with intelligence? Scientists have debated this question for almost two centuries now. Today, the matter may have been resolved: yes, bigger brains are more intelligent. Unconvinced? The present blog post reports the results of four articles that demonstrate the positive relationship between brain size and IQ. Though the studies are somewhat outdated, their findings represent interesting new developments in this question related to intelligence testing.
In In vivo brain size and intelligence, Willerman and colleagues reveal the first-ever study to correlate MRI-measured brain size and IQ in a healthy population (Wickett, Vernon, & Lee, 1994). Prior to the advent of new technologies such as magnetic resonance imaging (MRI), studies looking at brain size and intelligence were often performed post-mortem, i.e. using the brains of deceased individuals (Willerman et al., 1991). Since brains tend to shrink slightly with age, the results obtained were most likely inaccurate. In their study, Willerman et al. (1991) tested 40 college students using MRI and four subtests of the Wechsler Adult Intelligence Scale-Revised (WAIS-R): vocabulary, similarities, block design, and picture completion. They found inter alia that individuals with higher IQs had a larger brain size (r = 0.35, corrected) (Willerman et al., 1991). Factors such as body size and socioeconomic background were statistically controlled for (Willerman et al., 1991). The researchers’ conclusion was that human brain size is, in fact, positively correlated with performance on intelligence tests such as the WAIS-R (Willerman et al., 1991).
The results of the latter study were replicated by Andreasen et al. (1993), using the same measure (WAIS-R) and a sample of 67 adults undergoing MRI. Comparable to the previous study, brain size was correlated with IQ (r = 0.38). The following year, Wickett, Vernon, and Lee (1994) examined 40 right-handed women, and found that brain size and IQ correlated at r = 0.395. More recently, McDaniel (2005) published a meta-analysis which explored more deeply the question at hand. Based on 37 samples (1530 participants), the meta-analysis yielded a positive correlation of 0.33 between in vivo brain size and intelligence (McDaniel, 2005).
In light of the previous research, it seems that, indeed, humans who possess bigger brains are more intelligent. How can one explain these interesting results? While scientists are focused on examining brain scans, they seem to lack an explanation for them. Until they find a plausible account to back up their conclusion, the debate might never be completely resolved.
Student ID: 260536084
References
Andreasen, N. C., Flaum, M., Swayze II, V., O’Leary, D. S., Alliger, R., Cohen, G., Ehrhardt, J. & Yuh, W. T. C. (1993). Intelligence and brain structure in normal individuals. American Journal of Psychiatry. 150, 130–134.
McDaniel, M., A. (2005). Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence, 33, 337–346.
Wickett, J. C., Vernon, P. A., & Lee, D. H. (1994). In vivo brain size, head perimeter, and intelligence in a sample of healthy adult females. Personality and Individual differences, 16(6), 831–838.
Willerman, L., Schultz, R., Rutledge, J. N., & Bigler, E. D. (1991). In vivo brain size and intelligence. Intelligence, 15(2), 223–228.
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