One of the most influential studies in the field, published in 2001 by pioneering personality researchers Paul Costa, Robert McCrae and Antonio Terracciano, involved over 23,000 men and women from 26 cultures filling out personality questionnaires. Across these diverse cultures, including Hong Kong, USA, India and Russia, women consistently rated themselves as being warmer, friendlier and more anxious and sensitive to their feelings than did the men. The men, meanwhile, consistently rated themselves as being more assertive and open to new ideas. In the jargon of personality psychology, the women had scored higher on average on Agreeableness and Neuroticism and on one facet of Openness to Experience, while the men scored higher on one facet of Extraversion and a different facet of Openness to Experience.

Similar results came in 2008 when a separate research team asked more than 17,000 people from 55 cultures, to fill out personality questionnaires. Again, women scored themselves higher on Agreeableness and Neuroticism and this time also on Conscientiousness and the warmth and gregariousness facets of Extraversion.

One obvious criticism was that the participants were rating their own personalities. Perhaps the women and men differed simply because they were describing themselves in the way their societies expected them to be. But this seems unlikely because another study, led by McCrae and his collaborators, found broadly similar results from 12,000 people from 55 diverse cultures even though they were asked to rate the personality of a man or women they knew well, rather than their own personality.

Adding to the picture, other research has shown that the genders begin to differ in personality very early in life. For example, one study published in 2013 looked at ratings of the temperament of 357 pairs of twins made when they were three-years-old. The boys were rated as more active, on average, than the girls, while the girls were rated as more shy and as having more control over their attention and behaviour.