Finally, we created a composite score of twin-sharing of best friends by averaging the responses of both twins to the question: ‘how many of your three best friends are also your twin’s best friends?'. Possible responses to these questions ranged from 0 (none) to 3 (all 3). There was a significant association between twins as to the number of shared friends (r = 0.77, P < 0.001). This combined measure provided an index of shared friends for each twin pair.
