After completion of the survey items, some respondents were asked to administer a net-based battery of neuropsychological tests on spatial ability, memory and reading speed. Responses to these tests were obtained from 1,391 twins. Because many of the twins also had partners or spouses, we asked the twins to invite their partner to participate. This was done using a fully anonymous code, supplied in a closed envelope to the twin to be given onwards to the partner. Partner responses were obtained from 1,948 persons, spouse pair responses (i.e. both twin and partner replied) from 1,826 dyads. Some of the partners were also asked to take the neuropsychological tests, and 710 did so successfully, with 677 twin-partner dyads.
