We hypothesized that women with Turner syndrome (45,X) with a single X-chromosome inherited from their mother may show mentalizing deficits compared to women of normal karyotype with two X-chromosomes (46,X). Simple geometrical animation events (two triangles moving with apparent intention in relation to each other) which usually elicit mental-state descriptions in normally developing people, did not do so to the same extent in women with Turner syndrome. We then investigated the potential role in this deficit played by monoamine oxidase B enzymatic activity. MAO-B activity reflects central serotonergic activity, and by implication the functional integrity of neural circuits implicated in mentalizing. Platelet MAO-B was substantially reduced in Turner syndrome. However, contrary to prediction, in this (relatively small) sample there was no association between MAO-B enzymatic activity and mentalizing skills in participants with and without Turner syndrome.