Although monozygotic twins are genetically almost identical, a 2012 study of 92 pairs of monozygotic twins found that monozygotic twins acquire hundreds of genetic differences early in fetal development. This is caused by mutations (or copy errors) taking place in the DNA of each twin after the splitting of the embryo. Another study in 2018 of 450 pairs of monozygotic twins showed that often (87%) the two twins varied in sexuality. the study also showed that if one twin married before 40, the other was also likely to marry before 40.