Years of schooling did not function as a straightforward proxy for cognitive ability in a 2012 methamphetamine study. The researchers found that educational attainment and measured cognition related differently in the dependence and comparison groups. That observation matters when education is used to “correct” a cognitive comparison.
What population generated the finding?
Dean and colleagues studied 36 non-treatment-seeking participants with methamphetamine dependence and 42 healthy comparison participants in Los Angeles. The methamphetamine group reported 4–15 days of abstinence at testing and had negative testing for the examined drugs on assessment days, apart from nicotine. No experimental methamphetamine dose was administered.
The investigators measured several cognitive domains and constructed a composite score. They also assessed vocabulary knowledge as a proxy for academic learning. Cognitive performance, vocabulary and years enrolled in school were different variables, rather than interchangeable measures of intelligence.
The education model produced a discrepancy
Using a model developed in the comparison group, researchers predicted educational attainment from cognitive performance and demographic characteristics. The dependence group had, on average, approximately 1.05 fewer years of schooling than that model predicted; the paired comparison had p = 0.003.
This is a discrepancy relative to a study-specific model. It does not mean every person with methamphetamine dependence lost exactly one year of education, nor does it identify what any individual’s education would have been without exposure.
Earlier reported onset of methamphetamine use was associated with fewer years of education and a larger discrepancy between predicted and actual attainment. The cross-sectional design cannot establish whether methamphetamine caused that difference. Family circumstances, socioeconomic conditions and other developmental factors remain relevant alternatives.
Vocabulary raised a different measurement issue
The investigators estimated schooling obtained before the reported onset of methamphetamine use. Thirteen of the 36 participants had begun using before completing their education. The estimated pre-use schooling variable predicted vocabulary better than total schooling in their regression analysis.
That finding should not turn vocabulary into a perfect measure of educational quality. Vocabulary also reflects language development, social context and other abilities. The authors acknowledged that pre-existing verbal differences could help explain the pattern.
What about parental education?
Maternal education was explored as another correlate. In the dependence group, its correlation with cognitive performance was numerically stronger than the correlation involving participants’ own schooling. However, the study was underpowered to formally compare those correlations.
A significant correlation for one variable and a nonsignificant correlation for another do not, by themselves, establish a significant difference between the correlations. The exploratory result does not validate a general rule replacing a person’s cognitive assessment with a parent’s schooling.
Why this matters for interpretation
Before adjusting for education, identify its intended role. Is it meant to estimate prior cognitive ability, educational opportunity or acquired academic knowledge? Those are different constructs. A variable influenced by exposure and social circumstances may not behave as a neutral baseline correction.
The sample was small and selected, socioeconomic status was not comprehensively measured, and some participants overlapped earlier research cohorts. This paper should not be counted as wholly independent replication of every study it cites.
The defensible conclusion is methodological: in this historical sample, educational attainment was an imperfect proxy for the cognitive constructs being considered. It does not establish an individual’s premorbid ability, brain injury, current intoxication or impairment during an earlier event.
Primary scientific source
Dean AC, Hellemann G, Sugar CA, London ED. Educational Attainment is not a Good Proxy for Cognitive Function in Methamphetamine Dependence. Drug and Alcohol Dependence. 2012;123:249–254. doi:10.1016/j.drugalcdep.2011.11.019. Historical human observational study; original commentary on the supplied paper.
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