Saturday 7 February 2009

Social Class and IQ – some facts and statistics

Social Class and IQ – the facts and statistics

Mensa Magazine, December 2008

Bruce G Charlton

The Mensa magazine of October 2008 featured three articles on the subject of the relationship between Social Class and IQ. These were apparently prompted by the media coverage associated with my article on this topic published in the Times Higher Education online version:

It has been a bizarre experience to see myself so widely quoted as having views and holding opinions which bear no relation to what I wrote or believe! However, since the field of Social Class and IQ is important and frequently misunderstood, it seems worthwhile to use this opportunity to clarify some of the facts and statistics.

The research evidence for Social Class differences in IQ

The basic facts on Class and IQ are straightforward and have been known for about 100 years: higher Social Classes have significantly higher average IQ than lower Social Classes. For me to say this is simply to report the overwhelming consensus of many decades of published scientific research literature; so this information is neither new, nor is it just ‘my opinion’!

All the major scholars of intelligence agree that there are social class differences in IQ. As long ago as 1922, Professor Sir Godfrey Thomson and Professor Sir James Fitzjames Duff performed IQ tests on more than 13000 Northumbrian children aged 11-12, and found that the children of professionals had an average IQ of 112 compared with an average of 96 for unskilled labourers. These differences in IQ were predictive of future educational attainment.

Dozens of similar results have been reported since; indeed I am not aware of a single study which contradicts this finding. Social Class differences in intelligence are described in the authoritative textbook: IQ and Human Intelligence by N.J Mackintosh who is a Professor of Psychology at Cambridge University. And described in the 1996 American Psychological Association consensus statement Intelligence: knowns and unknowns:

Because IQ is substantially (although not entirely) hereditary (as has been shown by numerous studies of siblings including twins, and in adoption studies), and because IQ level is a good predictor of educational attainment; therefore with a fair system of exam-based selection, children from higher Social Classes will inevitably gain a disproportionately greater number of places at universities than those from lower Social Classes. And the more selective the university, then the higher will be the proportion of people from higher Social Classes compared with the proportion in the national population.

Statistical effects of Class IQ differences on Mensa qualification

Perhaps the best way to understand these statistics is to consider Mensa qualification.

Mensa is rather like a highly-selective university, because it admits only those people scoring in the top 2 percent of the UK population in a recognized IQ test. Indeed, Mensa imposes approximately the same degree of selection as Oxford and Cambridge Universities – although Oxbridge selects mostly on examination results rather than pure IQ. Exam results depend on a variety of factors as well as IQ, especially personality traits.

The average IQ of the UK population is defined as 100, with a standard deviation of 15. Mensa only accepts people with an IQ of about 130, or two standard deviations above average IQ.

But people in Social Class 4 & 5 – semi-skilled and unskilled workers – have an average IQ lower than 100: about 95 is a reasonable estimate. For people in Social Classes 1 and 2 (professional, managerial and technical – including teachers) the average IQ is higher than 100: about 110. (Reference: e.g. Hart et al. Public Health, 117, 187-195; I am using rounded numbers here for ease of calculation).

This means that there is approximately 15 IQ points, or one standard deviation, difference in average IQ across the Social Classes defined as above using the UK ‘Registrar General’ occupation-based system.

We can calculate that for semi- and un-skilled workers with an average IQ of 95, two standard deviations above the average gets us only to IQ 125. To qualify for Mensa a Social Class 4 & 5 person would therefore need to be two and one third standard deviations above the IQ average for their Class: so about 1 percent of Social Class 4 & 5 would qualify for Mensa.

And because Social Classes 1 & 2 have an average IQ of 110, then the Mensa threshold of IQ 130 is only 20 points above the average, or just one and a third standard deviations. This means that about 10 percent of Social Classes 1 & 2 would be expected to qualify for Mensa.

So a random person from Social Class 1 & 2 is about ten times as likely to qualify for Mensa as someone from the lowest Social Classes. Tenfold is a large difference.

The exact IQ of each Social Class depends upon how precisely the Social Classes are defined. The most educated and intellectual Social Classes (e.g. doctors, lawyers, chief executives of large corporations) have an average IQ about 130 – which means that about half the members of the most intellectually-selected Classes would be expected to qualify for Mensa. This proportion is about fifty times higher than the proportion of potential Mensans from semi- or un-skilled workers.

Social Class differences in attainment related to IQ should be expected

In conclusion, socioeconomic differences in average IQ are substantial and they will influence the proportions of people reaching specific levels of educational attainment or cognitive ability.

One common misunderstanding concerns averages. There is overlap in IQ between Social Class groups, and the situation is non-symmetrical for higher and lower Classes. People in jobs requiring high level skills or educational qualifications (e.g. architects or professional scientists) will almost-certainly all have above-average IQs. But a high IQ does not exclude people from unskilled jobs, and there will be a wider range of IQs in Social Classes 4 & 5. It is all a matter of percentages, not clear cut distinctions.

So, there will be some manual labourers who have higher IQs than some dentists, because it would be predicted that about one in a hundred labourers could get into Mensa while about half of dentists could not. But 130-plus IQ individuals will make-up a relatively small proportion of manual labourers compared with dentists.

Furthermore, the UK is mostly a middle class society nowadays. There are actually more people in Social Class 1 & 2 (around 40 percent of the working population) than there are in Classes 4 & 5 (about 20 percent). So in combination with the many-fold increased probability of higher IQ in higher Social Classes, this means that very selective organizations such as Mensa or Oxbridge should expect a fair and meritocratic selection mechanism to yield only a small proportion of people from the lowest Social Classes.

These facts and statistics are clearly unpopular in some quarters. Nonetheless, I feel that, given the overwhelming weight of evidence, we should now accept the reality of Social Class differences in IQ, and move-on to have a reasoned discussion of the implications.