Geneticists maintain that masculinity may be aided by female genes. An extra set of female genes appears to make males more masculine. This suggests that sex chromosomes, rather than hormones, have a role in directing behaviour, says Emilie Rissman, a behavioural neuroendocrinologist of the University of Virginia in America, following her 2012 study.

Rissman suggests that masculinity commences soon after birth. Male foetuses are exposed to testosterone from four weeks of age, while females are not.

Genetically, if an animal or human has XX chromosomes, it is female; if it has XY chromosomes, it is male.

To determine whether sex chromosomes affected sex-specific behaviours beyond dictating the presence of hormones, Rissman took advantage of a mutation in mice that causes the sex-determining region of the male Y chromosome to jump to a non-sex chromosome. The mice are male, but have two X chromosomes – that is, XX, which are typically female chromosomes.

While the XX male mice had the same level of testosterone as normal XY male mice, the XX male mice displayed more masculine sexual behaviours, such as mounting females more often and ejaculating more frequently.

To confirm that the differences were a result of a hidden factor on the X chromosome and not the lack of the Y chromosome, Rissman compared XY male mice with XXY male mice which carried an extra X chromosome. The result was that the XXY mice also showed more male sexual behaviours.

The extent to which these findings can be generalized and transferred to humans remains to be determined. However, the idea might provide an explanation for evidence that XXY men have more sex than men with the regular XY chromosomes. If Rissman can identify a part of the X chromosome linked to sexual activity, its protein products could be a target for libido-boosting therapies.

New Scientist, 10 March 2012

Photograph: Frey & Mosby, June 2015