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There are still many people who believe that intelligence does not exist or that it cannot be measured, particularly if the summary result is given as a single figure. The argument seems to be that single figure cannot possibly represent their myriad abilities. Quite so.
What are they to make of a recent finding by the Dunedin study team? This is an epidemiologically based study of child development in Dunedin, New Zealand, and it suggests that lead ingested in childhood is bad for intelligence and for later social mobility. If you maintain that intelligence does not exist, you need not be concerned.
March 28, 2017
Association of Childhood Blood Lead Levels with Cognitive Function and Socioeconomic Status at Age 38 Years and With IQ Change and Socioeconomic Mobility Between Childhood and Adulthood
Aaron Reuben; Avshalom Caspi; Daniel W. Belsky; et al
The authors say:
A prospective cohort study based on a population-representative 1972-1973 birth cohort from New Zealand; the Dunedin Multidisciplinary Health and Development Study observed participants to age 38 years (until December 2012).
Childhood lead exposure ascertained as blood lead levels measured at age 11years. High blood lead levels were observed among children from all socioeconomic status levels in this cohort.
MAIN OUTCOMES AND MEASURES
The IQ (primary outcome) and indexes of Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed (secondary outcomes) were assessed at age 38 years using the Wechsler Adult Intelligence Scale–IV (WAIS-IV; IQ range, 40-160). Socioeconomic status (primary outcome) was assessed at age 38 years using the New Zealand Socioeconomic Index-2006 (NZSEI-06; range, 10 [lowest]-90 [highest]).
Of 1037 original participants, 1007 were alive at age 38 years, of whom 565 (56%) had been lead tested at age 11 years (54% male; 93% white). Mean (SD) blood lead level at age 11 years was 10.99 (4.63) μg/dL. Among blood-tested participants included at age 38 years, mean WAIS-IV score was 101.16 (14.82) and mean NZSEI-06 score was 49.75 (17.12). After adjusting for maternal IQ, childhood IQ, and childhood socioeconomic status, each 5-μg/dL higher level of blood lead in childhood was associated with a 1.61-point lower score (95% CI, −2.48 to −0.74) in adult IQ, a 2.07-point lower score (95% CI, −3.14 to −1.01) in perceptual reasoning, and a 1.26-point lower score (95% CI, −2.38 to −0.14) in working memory. Associations of childhood blood lead level with deficits in verbal comprehension and processing speed were not statistically significant. After adjusting for confounders, each 5-μg/dL higher level of blood lead in childhood was associated with a 1.79-unit lower score (95% CI, −3.17 to −0.40) in socioeconomic status. An association between greater blood lead levels and a decline in IQ and socioeconomic status from childhood to adulthood was observed with 40% of the association with downward mobility mediated by cognitive decline from childhood.
CONCLUSIONS AND RELEVANCE
In this cohort born in New Zealand in 1972-1973, childhood lead exposure was associated with lower cognitive function and socioeconomic status at age 38 years and with declines in IQ and with downward social mobility. Childhood lead exposure may have long-term ramifications. JAMA. 2017;317(12):1244-1251. doi: 10.1001/jama.2017.1712
The big advantage of this sample is that it is very well drawn up to be representative of the New Zealand population, and by implication representative of European populations. It has been closely studied and assiduously followed up. Happily, we also have a good genetic study of the participants, so it is apposite to give those results. Sorry for this nested parenthetical approach, but it provides context.
The Genetics of Success: How Single-Nucleotide Polymorphisms Associated With Educational Attainment Relate to Life-Course Development.
Daniel W. Belsky, Terrie E. Moffitt, David L. Corcoran, Benjamin Domingue, HonaLee Harrington, Sean Hogan, Renate Houts, Sahttp://www.paulgraham.com/say.htmlendhya Ramrakha, Karen Sugden, Benjamin S. Williams, Richie Poulton, Avshalom Caspi.
June 1, 2016. Psychological Science Vol 27, Issue 7, 2016
A previous genome-wide association study (GWAS) of more than 100,000 individuals identified molecular-genetic predictors of educational attainment. We undertook in-depth life-course investigation of the polygenic score derived from this GWAS using the four-decade Dunedin Study (N = 918). There were five main findings. First, polygenic scores predicted adult economic outcomes even after accounting for educational attainments. Second, genes and environments were correlated: Children with higher polygenic scores were born into better-off homes. Third, children’s polygenic scores predicted their adult outcomes even when analyses accounted for their social-class origins; social-mobility analysis showed that children with higher polygenic scores were more upwardly mobile than children with lower scores. Fourth, polygenic scores predicted behavior across the life course, from early acquisition of speech and reading skills through geographic mobility and mate choice and on to financial planning for retirement. Fifth, polygenic-score associations were mediated by psychological characteristics, including intelligence, self-control, and interpersonal skill. Effect sizes were small. Factors connecting DNA sequence with life outcomes may provide targets for interventions to promote population-wide positive development.
Of course, this is the sort of snappy title and genetic finding which drives some people to doubt the existence of intelligence. It is against this background that the neurotoxic effects of lead are interesting. Crucially, lead exposure in New Zealand in the 1980s did not follow a social class gradient. It mostly came from car exhausts, and was thus an equal opportunity toxin.
Of 1037 participants in the original cohort, 1007 were still alive at age 38 years, 565 (56%) of whom had been lead tested at age 11 years (303 [54%] male; 525 [93%] white). Participants alive at age 38 years with childhood blood lead data (n=565) and without childhood blood lead data (n=442) did not differ to a statistically significant extent from each other in terms of their mothers’ IQ scores or their social class origins, but those without blood lead data did have lower mean childhood IQ scores as a group.
The correlation between childhood blood level and age 38 IQ is merely -.11 which is the sort of size I usually ignore, yet the mean differences are instructive, as shown above.
Table 2 is interesting at many levels. Maternal IQ, tested on verbal material only, correlates at r= 0.38 with child IQ, and rises to r= 0.44 at age 38, long after the child has left home, which is the typical pattern. Childhood IQ is a better predictor of socio-economic status at age 38 (r= 0.43) than childhood economic status (r= 0.35).
The net result is that after controlling for participants’ own childhood IQ score, their mothers’ IQ score, and their socioeconomic background, each 5-μg/dL higher level of blood lead in childhood was associated with an additional 1.61-point lower score (95% CI, −2.48 to −0.74; P<.001) in the full scale IQ.
Children with higher levels of blood lead at age 11 years scored lowest on indexes tapping perceptual reasoning and working memory.
First, childhood blood lead level was associated with lower adult IQ scores nearly 3 decades later, reflecting cognitive decline following childhood lead exposure. There were significant associations between childhood blood lead levels and lower scores on the Perceptual Reasoning IQ and the Working Memory IQ, but no significant association with the Verbal Comprehension IQ or the Processing Speed IQ. These associations remained significant after adjusting for the participants’ childhood IQs, their mothers’ IQs, and their social class backgrounds.
Second, childhood blood lead level was associated with lower adult socioeconomic status, reflecting downward social mobility following childhood lead exposure. These associations too remained significant after adjusting for the participants’ childhood IQs, their mothers’ IQs, and their social class backgrounds.
Third, the relationship between childhood lead exposure and downward social mobility by midlife was partially but significantly mediated by cognitive decline following childhood lead exposure.
In conclusion, this study shows how a neuro-toxin can have an effect on intelligence, of similar magnitude to low birth weight. By the way, intelligence can be influenced by the environment despite having a large genetic component. By the way, the effect shows up on the “single figure” of Wechsler Full Scale IQ, and by looking at the 10 subtests you can identify which abilities are most affected. The exposure to lead decreases ability (one figure) and thereby reduces achieved socio-economic status (one figure). No surprise for us, but possibly a surprise to some sociologists. The effect is not uniform, and it is not clear why something like Processing Speed is unaffected. Processing speed certainly slows with age and is often poor in children with brain related disorders. This is minor puzzle in the general picture, but since this is an excellent representative sample it is more likely to be right than the necessarily selective clinical brain damage samples.
Final word: if someone tells you they do not believe in intelligence reply that you wish them well, but that if they have children they should keep them well away from neuro-toxins because, among other things, they reduce social mobility.