IQ and Physical Health
But, peer reviewed research links IQ to poor physical health outcomes for everyone, and to anti-social behavior in boys. A study of the entire population of Scotland, for example, found that (citations omitted):
[A] drop of 1 standard deviation in IQ was associated with a 27% increase in cancer deaths among men and a 40% increase in cancer deaths among women. The effect was especially pronounced for stomach and lung cancers, which are specifically associated with low socioeconomic status (SES) in childhood.
For each standard deviation increase in IQ, there was a 33% increased rate of quitting smoking. Adjusting for social class reduced this rate only mildly, to 25%. Thus, childhood IQ was not associated with starting smoking (mostly in the 1930s, when the public were not aware of health risks), but was associated with giving up smoking as health risks became evident.
When all other variables were statistically controlled, each additional IQ point predicted a 1% decrease in risk of death. Also, IQ was the best predictor of the major cause of death, motor vehicle accidents. Vehicular death rates doubled and then tripled at successively lower IQ ranges (100–115, 85–100, 80–85).
Scottish data from a study of 1145 men and women over twenty years also show a strong link between IQ and cardivascular disease deaths (i.e. heart attack and stroke). IQ predicts death from cardivascular disease better than systolic blood pressure or physical activity and was the second most powerful predictor of overall mortality risk:
[T]he relative index of inequality (sex-adjusted hazard ratio, 95% confidence interval) for the most disadvantaged relative to the advantaged persons was (in descending order of magnitude for the top five risk factors): 5.58 (2.89, 10.8) for cigarette smoking; 3.76 (2.14, 6.61) for IQ; 3.20 (1.85, 5.54) for income; 2.61 (1.49, 4.57) for systolic blood pressure and 2.06 (1.07, 3.99) for physical activity. Mutual adjustment led to some attenuation of these relationships. Similar observations were made in the analyses featuring all deaths where, again, IQ was the second most powerful predictor of mortality risk.
Also, recall that in Scotland, they have universal health care. The IQ impact on physical health in Scotland is not simply a function of a reduced ability of low IQ individuals to access health care providers. The study's authors note that disparity in health outcomes by social class actually increases (despite improvement for all) as access to health care becomes more widespread. They theorize that this disparity is largely a result of an inability to follow doctors' orders and recommendations due to functional illiteracy and an inability to quickly learn the disease and injury management skills that they are taught. This suggests that the system needs to dumb down, focus and follow up on the way it communicates critical information to patients, at least where there is reasons to doubt that they really understand what they are being told.
A study of 4,166 U.S. soldiers likewise found that "lower IQ scores in both early adulthood and middle age were related to total and CVD mortality at a level of magnitude greater than many traditional risk indices."
Part of this is because "smokers have a much lower average IQ, 7.5 points lower" than non-smokers (about half of a standard deviation), but the strength of the relationship between IQ and physical health is clearly much broader than making bad decisions about smoking.
IQ, Gender, ADHD and Childhood Anti-Social Behavior
Finally, a large twin study looked at the link between IQ, anti-social behavior in young children, gender, and ADHD:
Children’s IQ was assessed via individual testing at age 5 years. Mothers and teachers reported on children’s antisocial behavior at ages 5 and 7 years. Low IQ was related to antisocial behavior at age 5 years and predicted relatively higher antisocial behavior scores at age 7 years when antisocial behavior at age 5 years was controlled. This association was significantly stronger among boys than among girls. Genetic influences common to both phenotypes explained 100% of the low IQ–antisocial behavior relation in boys.
The link between anti-social behavior and IQ is limited more or less exclusively to boys. "The prevalence of this research diagnosis of ADHD was 8% (70% boys; 30% girls). . . the genders scored the same for IQ." Excluding children diagnosed with ADHD did not change the extremely strong relationship between IQ and anti-social behavior in young boys, but once ADHD was removed as a factor, there was no significant relationship between IQ and anti-social behavior in girls (same study, citations omitted):
The exclusion of children who received a diagnosis of ADHD had no effect on our findings regarding the gender difference in the strength of the association between low IQ and antisocial behavior. . . .
[O]nce children with ADHD diagnoses were excluded from the sample, the low IQ–antisocial behavior correlation in girls was attenuated to almost nonsignificance. These findings suggest that the low IQ–antisocial behavior relation in girls is largely an artifact of comorbid ADHD.
The population prevalence of early-onset antisocial behavior that is life-course persistent is low (5% among men, less than 1% among women); however, these individuals account for more than their share of crime. Low IQ predicts the chronicity of antisocial behavior; therefore, the children in our study who are boys, have low IQs, and have high levels of early antisocial behavior are at high risk for becoming life-course persistent antisocial individuals. This antisocial subtype is at the highest risk for myriad negative outcomes in adulthood, including mental health problems, substance dependence, financial problems, drug-related violent crime, and violence against women and children.
Genetic influences on IQ and antisocial behavior suggest that the parents of these vulnerable children are also likely to have low IQ and to be antisocial. Such parents are at risk for creating family environments that aggravate rather than ameliorate their children’s vulnerabilities. Thus, the families of young boys with low IQ who exhibit high levels of antisocial behavior should be targeted for early intervention.
This news is discouraging. Life is not fair.
Adult IQ has a large hereditary component and to the extent that IQ has an environmental component it is stubbornly resistant to change by the time that one is an adult. "Get smarter" isn't a very helpful recommendation for your physician to give you.
It is also discouraging that a significant number of young boys, one or two in every elementary school classroom, on average, appear to be already firmly on a path to lifetime failure and high risk for criminal activity by the time that they start kindergarten. The myth of the dumb bully is apparently more than a myth. IQ completely explains, by itself, your likelihood of being a bully if you are a boy.
The study also corroborates the anecdotal observation that good conduct in children is correlated with academic competence.
Doubts about the extent to which IQ is really environmental as opposed to genetic are also beside the point. It isn't practicable or consistent with the way we have organized our society to take little boys away from parents because the children have low IQs or are anti-social in kindergarten. So, even if the problem is the environment, it is an environment that isn't within our power to change very much.
UPDATE: After reading the underlying study more closely, I'm adding a quote that clarifies matters:
Genetic influences in common with IQ accounted for 12% of the variance in antisocial behavior. The remaining variance in antisocial behavior was accounted for by genetic influences (61%) and nonshared environmental influences (27%) unique to antisocial behavior. These results indicate that the etiology of the low IQ–antisocial behavior relation in boys cannot be explained by the ADHD diagnosis. However, the exclusion of children with the ADHD diagnosis further attenuated the already small IQ–antisocial behavior relation in girls.
Thus, the point of the study on IQ, ADHD and anti-social behavior is not that IQ is the primary determinant of anti-social behavior (IQ accounts for only 12% of anti-social behavior variance in young boys). Instead, it shows that in boys, the influence of IQ on anti-social behavior is limited to their common genetic source, not to any environmental factors that influence IQ. But, IQ has no impact on anti-social behavior in young girls.
Rather than re-write my analysis, I'll note that the results, while not as clear in linking IQ and anti-social behavior as I'd understood in reading only a summary of the study, still point to a very strong genetic basis for anti-social behavior in young boys.
Overall, 73% of variance in anti-social behavior in young boys can be attributed to genetic causes. This is huge. Very few social or psychological traits are more than 50% genetic. And, the observation that impacts from a shared environment (if any) are for practical policy purposes almost as hard to change as impacts from genetic causes still holds.
It is also worth noting that the study on IQ, ADHD and anti-social behavior drew its conclusions largely from variations in IQ within the normal range, not just from mental retardation per se. The relationship between anti-social behavior and IQ was stronger however, at the lower end of the scale.
Another twin study, looking at children as they grow older, also distinguishes between two kinds of anti-social behavior, one having a greater genetic component and more stability over time than the other:
In childhood, aggressive ASB was highly heritable and showed little influence of shared environment, whereas nonaggressive ASB was significantly influenced both by genes and shared environment. In adolescence, both variables were influenced both by genes and shared envirnmment. The continuity in aggressive antisocial behavior symptoms from childhood to adolescence was largely mediated by genetic influences, whereas continuity in nonaggressive antisocial behavior was mediated both by the shared environment and genetic influences. These data are in agreement with the hypothesis that aggressive ASB is a stable heritable trait as compared to nonaggressive behavior, which is more strongly influenced by the environment and shows less genetic stability over time.
This is not the only study in the literature to distinguish between aggresive antisocial behavior and nonaggressive antisocial behavior and suggest that they have different causes. A 1999 study reaches essentially the same conclusions. A 2004 paper notes that:
Non-shared environment and genetic factors substantially influenced both forms of ASB. The heritability of aggressive (but not non-aggressive) ASB was significantly higher in girls than in boys. Combining both sexes, a model in which the genetic effects on aggressive and non-aggressive ASB were identical could be rejected. Our results suggest a partial genetic overlap with a specific genetic effect contributing to the variance of aggressive ASB and a stronger genetic effect on aggression in females than in males.
Yet another study looks at the connection between psychopathy and anti-social behavior. It found that the genetic causes of anti-social behavior are essentially the same as the more narrow situation of psychopathy (roughly speaking a lack of a conscience). Shared environmental factors further influenced antisocial behavior, but not psychopathy. Thus, some people are born without a conscience and that isn't caused by bad parenting or schooling, but bad parenting or schooling can be a separate cause of antisocial behavior.
So what is kind of genes are associated with the 61% of variation in anti-social behavior attributable to non-IQ, non-ADHD genese in boys? Those appear to be the same genes associated with psychopathy.
A common genetic factor loaded substantially on both psychopathic personality traits and antisocial behavior, whereas a common shared environmental factor loaded exclusively on antisocial behavior.
CONCLUSIONS: The genetic overlap between psychopathic personality traits and antisocial behavior may reflect a genetic vulnerability to externalizing psychopathology. The finding of shared environmental influences only in antisocial behavior suggests an etiological distinction between psychopathic personality dimensions and antisocial behavior. Knowledge about temperamental correlates to antisocial behavior is important for identification of susceptibility genes, as well as for possible prevention through identification of at-risk children early in life.
Another study links the genetic factors behind anti-social behavior to mental traits more familiar than psychopathy, i.e. spatial and memory functions:
Antisocial behavior . . . [clusters into] 4 groups: control, childhood-limited, adolescent-limited, and life-course persistent. Those on the lifecourse persistent path and also on the childhood-limited path were particularly impaired on spatial and memory functions. Impairments were independent of abuse, psychosocial adversity, head injury, and hyperactivity. Findings provide some support for the life-course persistent versus adolescent-limited theory of antisocial behavior and suggest that (a) neurocognitive impairments are profound and not artifactual and (b) childhood-limited antisocials may not be free of long-lasting functional impairment.
This is at odds with theories that have suspects that insufficient verbal ability to resolve disputes effectively is a key factor in anti-social behavior, which appears to have the pattern of causation backwards. According to a 2002 study:
Persistently antisocial individuals (N = 47) had spatial deficits in the absence of verbal deficits at age 3 years compared to comparisons (N = 133), and also spatial and verbal deficits at age 11 years. Age 3 spatial deficits were independent of social adversity, early hyperactivity, poor test motivation, poor test comprehension, and social discomfort during testing, and they were found in females as well as males. Findings suggest that early spatial deficits contribute to persistent antisocial behavior whereas verbal deficits are developmentally acquired.