1. Nylander PPS Frequency of multiple births,1975
2. Epidemiological trends in multiple births in the United States, 1971–1998;Kiely;Twin Res,2001
3. Twinning rates;Murphy;Lancet,1997
4. Twins;Phelan,1993
5. The history of twins, as a criterion of the relative powers of nature and nurture;Galton;J Anthropol Inst,1875