Figure 2.

Correlation with respect to abstinence among 16-year-old identical (i.e., monozygotic [MZ]) and fraternal (i.e., dizygotic [DZ]) Finnish twin brothers living in the greater Helsinki area or in rural areas in northern Finland. In the Helsinki area, where abstinence rates among 16-year-olds are about half as high as in northern Finland, the twins’ zygosity strongly influenced the degree of correlation for abstinence. In northern Finland, no difference between MZ and DZ twin brothers existed.

SSDZ = same-sex dizygotic.

SOURCE: Rose et al. 1998.