Figure 4.

Standardized mean scores for 14 spatial ability scores, separately for four twin groups, based on the sex of the twin and co-twin. Note. Mm = Males with a male co-twin; Mf = Males with a female co-twin; Fm = Females with a male co-twin; Ff = Females with a female co-twin. All the tests were standardized for the whole sample (males and females combined) with a mean of 0. Analyses were then run on these standardized values for males and females separately. For all measures, males’ average scores were positive and females’ average scores were negative.”