Sex differences in human intelligence have long been a topic of debate among researchers and scholars. It is now recognized that there are no significant sex differences in general intelligence, though particular subtypes of intelligence vary somewhat between sexes.
While some test batteries show slightly greater intelligence in males, others show slightly greater intelligence in females. In particular, studies have shown female subjects performing better on tasks related to verbal ability, and males performing better on tasks related to rotation of objects in space, often categorized as spatial ability.
Some research indicates that male advantages on some cognitive tests are minimized when controlling for socioeconomic factors. It has also been hypothesized that there is slightly higher variability in male scores in certain areas compared to female scores, leading to males' being over-represented at the top and bottom extremes of the distribution, though the evidence for this hypothesis is inconclusive.
IQ research
Background
There is no statistically significant difference between the average IQ scores of men and women. Average differences have been reported, however, on some tests of mathematics and verbal ability in certain contexts. Some studies have suggested that there may be more variability in cognitive ability among males than among females, but others have contradicted this, or presented evidence that differential variability is culturally rather than biologically determined. According to psychologist Diane Halpern, "there are both differences and similarities in the cognitive abilities of women and men, but there is no data-based rationale to support the idea that either is the smarter or superior sex."
Findings
Although most tests show no sex difference, there are some that do. For example, it has been found that female subjects tend to perform better on tests of verbal abilities and processing speed while males tend to perform better on tests of visual-spatial ability and crystallized intelligence. For verbal fluency, females have been specifically found to perform slightly better, on average, in vocabulary and reading comprehension and significantly higher in speech production and essay writing. Males have been specifically found to perform better, on average, in spatial visualization, spatial perception, and mental rotation. None of these findings, however, suggest an advantage for either sex in general intelligence, nor in fluid intelligence.
Some research has suggested that there is a very small IQ advantage for males in some countries, ranging from two to three points in the United States and Canada up to around four points in China and Japan. On the other hand, other research suggests that there is an IQ advantage in favor of females.
Most studies find either a very small difference or no sex difference with regard to general intelligence. In 2000, researchers Roberto Colom and Francisco J. Abad conducted a large study of 10,475 adults on five IQ tests taken from the Primary Mental Abilities and found negligible or no significant sex differences. The tests conducted were on vocabulary, spatial rotation, verbal fluency and inductive reasoning.
The literature on sex differences in intelligence has produced inconsistent results due to the type of testing used, and this has resulted in debate among researchers. Garcia (2002) argues that there might be a small insignificant sex difference in intelligence in general (IQ) but this may not necessarily reflect a sex difference in general intelligence or g factor. Although most researchers distinguish between g and IQ, those that argued for greater male intelligence asserted that IQ and g are synonymous (Lynn & Irwing 2004) and so the real division comes from defining IQ in relation to g factor. In 2008, Lynn and Irwing proposed that since working memory ability correlates highest with g factor, researchers would have no choice but to accept greater male intelligence if differences on working memory tasks are found. As a result, a neuroimaging study published by Schmidt (2009) conducted an investigation into this proposal by measuring sex differences on an n-back working memory task. The results found no sex difference in working memory capacity, thus contradicting the position put forward by Lynn and Irwing (2008) and more in line with those arguing for no sex differences in intelligence.