Why are some twins so close as to develop something approximating a private language (called idioglossia or cryptophasia) while others are connected only by animosity? For a look-alike conversation between infant twins see this very funny clip.
Are identical twins close more often than fraternal twins? If so, does this suggest that the determining factor for closeness may be genetic rather than parental?
In fact, twin research does show that identical twins are closer than fraternal twins, even when reared apart. Furthermore, identical twins tend to feel closer to the twin they may have only recently met than to adoptive siblings with whom they were raised. Researchers assume a genetic, evolutionary explanation, but physical resemblance and personality similarities may play a role as well. Hopefully, further research will help explain the mechanism by which genetics influences complex human behavior.
I can’t help but feel that even these suggestive findings are hopeful news for parents who fear they may have unwittingly stimulated animosity between their children.