We first examined whether MZ twins more often share
their friends than DZ twins. Table 1 shows that MZ twins
have significantly more often all their friends in common
than DZ twins (v2(8, n = 1526) = 136.79, P.001).
About 20% of the MZ twins (20.3% in the MZM and
18.2% in the MZF twins) share all their friends. In DZ
same-sex twins these percentages were substantially lower
(i.e., 4.7% in DZM and 3.4% in DZF) and it was particularly
low in DZ opposite-sex twin pairs (1.1%).