A Matter of Heredity IQ of Parents
Intelligence? A matter of organic chemistry. We are not responsible for being more intelligent than be stupid.
In terms of IQ, the more stinging questions revolve around the heritability (in the genetic sense). In 1994 appeared The Bell Curve, a book co-written by an anthropologist (Charles Murray) and a psychologist (Richard Herrnstein). The book contains nearly a century of research on IQ and reaffirms that the best estimator is now known as “social success” in the United States. Many readers have understood this as a justification for social inequalities. Charles Murray (Herrnstein died in 1994) refutes this interpretation, for him, the text simply repeats the known data, and merely describes reality.
Anyway, a real rage followed the publication of this book. It is this question that awakens in us the reactions of defense against the risk of both justification of inequality and discrimination by money: why the fight against social reproduction that the son of lawyer laborers mostly lawyers and conversely, if it stems from genetics? It is viscerally that some people tend to defend the idea of intelligence primarily acquired to give a chance to our values and sense of school. Especially as we keep in mind drifts hereditary theories of intelligence that can be used at once eugenics, the extermination of the so-called idiots, or the justification of social inequalities. Do not forget, however, that the inverse theory – a psychic functioning almost entirely acquired – led some psychoanalysts to condemn mothers of autistic children.
Heritability of IQ
For over a century the study of intelligence started and never stopped. In total, more than 200 000 people were tested, and the results, well summarized by Jacques Bénesteau are clear: IQ is influenced by genes, at least as much as by nongenetic factors.
The case of Cyril Burt
Cyril Burt, British psychologist, was considered until his death in 1971 as a serious scientist, and a pioneer in research on intelligence. He published three surveys IQ of monozygotic twins reared apart from birth, the first of which dates from 1955 and includes 21 pairs of monozygotic twins. He announced a correlation coefficient of 0.54 for dizygotic twins and 0.77 for monozygotic. This research was greeted with enthusiasm by some people, including supporters of theses … morally questionable and rejected by others, for reasons as unscientific, but much more compatible with modern ethics.
Burt, convinced of the reality of its first results (which will be independently confirmed later) publishes two other studies. One in 1958 over 30 pairs of monozygotic twins. The other, in 1966, of 53 pairs. It was only after his death that discovered the pot with roses: some of these subjects apparently never existed. It seems that the researcher has also invented the IQ of parents in some cases … and even her staff ghosts Jane Conway and Margaret Howard, as was revealed in the Sunday Times of 24 October 1976.
This fraud has been used by some to question the validity and integrity of all twin studies estimating the heritability of IQ. Yet dozens of independent studies have replicated the results obtained so Burt was partly fraudulent.
To determine more or less hereditary IQ, we studied children adopted before 6 months. Their IQ depends weakly than their adoptive parents when they have 2 or 3 years (typically r = 0.15), but this association decreases very rapidly, reaching 0-40 years. In other words, the IQ of adoptive parents did “effect” in early childhood. Instead, the link between children’s IQ and that of the biological parents is increasing in life, reaching about 0.45 to adulthood, even when children do not know their biological parents.
The correlation coefficient r measures the relationship between two variables. It is 0 when there is no relationship, and 1 case of perfect positive correlation. It is assumed that r shows a link from moderate 0.5 and a strong bond around 0.8. The square of this coefficient can be interpreted as a proportion of variation explained.
For example, the correlation between IQ mothers and their children is typically 0.45, which means that about 20% of variations in children’s IQ can be explained by variations in maternal IQ (which not mean that these 20% are due to genes).
The study of twins was also rich in information: dizygotic twins (“twins”) are, in terms of IQ, just closer than non-twin brothers. Regarding monozygotic (“identical twins”), who share their genetic heritage, the correlation reaches 0.86 in the case where they lived together and slightly less otherwise. 0.86 is about the test-retest correlation, which is obtained by measuring the same people twice.
Studies of mild mental retardation and precocious children confirm these results. Studies give different correlations with the age of subjects, but heritability (proportion of variation in IQ due to genes) never drops below 40%, and in adulthood is often estimated at 80%. Although this percentage is a delicate and potentially misleading interpretation can be summarized as the thousands of studies on the question of the heritability of IQ: IQ to know a person, the most valuable information is IQ of his parents, not his family or educational environment, although in practice the two are often correlated. The heritability of IQ is not explained by the success of social workers to have high IQ children rather high IQ, low IQ lawyers have rather low IQ children.
52 experts wrote in The Wall Street Journal
After the publication of the book The beautiful curve, the authors have suffered a barrage of harsh criticism. Some of these criticisms from journalists rather than psychologists, attacking not only the opinions (real or imagined) of Murray and Herrnstein on inequality, but also the fact that IQ can be largely hereditary, and is a effective predictor of social success.
It is for this reason that the IQs 52 experts, many of whom are opposed to the use of IQ, drafted a manifesto in 25 points, to reaffirm the current knowledge on the heritability of IQ. They do not necessarily endorse any political position based on these results. Part of psychologists asked to sign refused to do so, while asserting agree with the bottom, for reasons of possible interpretations of the text. Some disagreed with one or other of the 25 points. The article was published December 13, 1994 in The Wall Street Journal.