Summary of demographic variables of study participants.

*Participants' parent's professions were assigned to categories specified by the Office of National Statistics (ONS). Occupational data from the ONS were used as a reference and the gender-specific median annual full-time London earnings of the relevant category were assigned to each parent. The earnings of both parents were added together to obtain the family income. For the partial correlation analyses, the Hollingshead four factor index [20] was derived from individual earnings (as above) for each parent and the index for the parent with the highest earnings was used as a measure of family income. The ONS table used to calculate family income was titled: PROV - Work Region Occupation SOC10 (2) Table 3.7a Annual pay - Gross 2011. Summary of demographic variables of study participants.