Mild to severe mental retardation is a symptom of several hundred single-gene disorders and many chromosomal abnormalities, including small deletions. Based on twin studies, moderate to severe mental retardation does not appear to be familial (run in families), but mild mental retardation does. That is, the relatives of the moderate to severely mentally retarded have normal IQs, whereas the families of the mildly mentally retarded have low IQ.

IQ score ranges (from DSM-IV)

1. mild mental retardation: IQ 50-55 to 70; children require mild support; formally called "Educable Mentally Retarded"
2. moderate retardation: IQ 35-40 to 50-55; children require moderate supervision and assistance; formally called "Trainable Mentally Retarded"
3. severe mental retardation: IQ 20-25 to 35-40; can be taught basic life skills and simple tasks with supervision
4. profound mental retardation: IQ below 20-25; usually caused by a neurological condition; require constant care

The CDC listed the prevalence of mental retardation across race and gender in a 1991 study: overall, the rate of mental retardation among blacks was 16.6%, compared to 6.8% for whites. Notable is that blacks were almost three times as likely to be labeled mildly mentally retarded as compared to whites. Mild mental retardation is almost never diagnosed until a person enters elementary school, which critics claim lends strong support to the notion that IQ tests are racially biased.

IQ, education, and Income:

Tambs et al (1989) found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment ... contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ". In a sample of US siblings, Rowe et al (1997) report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.

Regression:

The heritability of IQ determines the extent to which the IQ of children will be similiar to the IQ of parents. Because the heritability of IQ is less than 100%, the IQ of children tends to "regress" towards the mean IQ of the population. That is, high IQ parents tend to have children who are less bright than their parents, whereas low IQ parents tend to have children who are brighter than their parents. The effect can be quantified by the equation where:

is the predicted average IQ of Mom and Dad's children
is the mean IQ of the population that Mom and Dad come from h² is the heritability of IQ
Thus, if the heritability of IQ is 50%, a couple with an average IQ of 120 will have children that average around an IQ of 110.

Brain size and IQ:

Modern studies using MRI imaging have shown that brain size correlates with IQ by a factor of roughly .35 to .40. In 1991, Willerman et al. used data from 40 White American university students and reported a correlation coefficient of .35. Other studies done on samples of Caucasians show similar results, with Andreasen et al (1993) determining a correlation of .38, while Raz et al (1993) obtained a figure of .43 and Wickett et al (1994) obtained a figure of .40. The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no within family correlation (Schoenemann et al. 2000). A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002).

The Flynn effect

Worldwide, IQ scores appear to be slowly rising, a trend known as the Flynn effect, so that tests need repeated renormalization.

Sex and IQ:

Most IQ tests are designed so that the average IQs of males and females are equal. However, men tend to score higher in the parts of the test that cover spatial and quantitative abilities, and women generally score higher in the verbal sections. Some research has shown that the variance in men's IQ scores is greater than the variance among women's, as seen in other cognitive test scores. This would mean that men are more likely than women to have both very high and very low IQs.

Religiousness and IQ:

Some studies claim a correlation between higher IQ, SAT scores, GPA and degree of religious belief. Such claims merit skepticism - especially due to the psychological, sociological and cultural motivations that often prompt such research. While most of the research indicates a negative correlation between IQ and religiousness, this is open to debate.

Health and IQ:

Persons with a higher IQ have generally lower adult morbidity and mortality. This may be because they better avoid injury and take better care of their own health. It also decreases the risk of Post-Traumatic Stress Disorder, severe depression, and schizophrenia. On the other hand, it increases the risk of Obsessive Compulsive Disorder [2].

Research in Scotland has shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.