Anderson, W. J. R. (1956) Stillbirth and neonatal mortality in twin pregnancy. J. Obstet. Gynaec. Br. Emp. 63, 205.
Armitage, P. (1971) Statistical Methods in Medical Research. Blackwell, Oxford.
Bishop, Y. M. M., Feinberg, S. E., Holland, P. W. (1975) Discrete Multivariable Analyses: Theory and Practice. MIT Press, Cambridge, Massachusetts.
Bønnelykke, B., Olsen, J. & Nielsen, J. (1990) Coital frequency and twinning. J. biosoc. Sci. 22, 193.
Campbell, D. M., Campbell, A. J. & MacGillivray, I. (1974) Maternal characteristics of women having twin pregnancies. J. biosoc. Sci. 6, 463.
Corney, G., Seedburgh, D., Thompson, B., Campbell, D. M., MacGillivray, I. & Timlin, D.(1979) Maternal height and twinning. Ann. hum. Genet. 43, 55.
Elwood, J. M. (1973) Changes in the twinning rate in Canada 1926–70. Br. J. prev. soc. Med. 27,236.
Hansen, E. J. (1984) Socialgrupper i Danmark. SFI Studie 48, Socialforskningsinstituttet, Copenhagen.
Hemon, D., Berger, C., Lazar, P. (1979) The etiology of human dizygotic twinning with special reference to spontaneous abortions. Acta Genet. med. Gemell. 28, 253.
James, W. H. (1972) Secular changes in dizygotic twinning rates. J. biosoc. Sci. 4, 427.
Jeanneret, O. & McMahon, B. (1962) Secular changes in rates of multiple births in the United States. Am. J. hum. Genet. 14, 410.
Lilienfeld, A. M. & Pasamanick, B. (1955) A study of variations in the frequency of twin births by race and socioeconomic status. Am. J. hum. Genet. 7, 204.
McArthur, J. W. (1942) Relations of body size to litter size and to the incidence of fraternal twins.J. Hered. 33, 87.
Nielsen, J., Homma, A., Isaksen, B. & Bertelsen, A. (1978) Incidence of twin births in Denmark from 1911 to 1974. Acta Genet. med. Gemell. 27, 45.
Nylander, P. P. S. (1981) The factors that influence twinning rates. Acta Genet. med. Gemell. 30,189.
Rachootin, P. & Olsen, J. (1980) Secular changes in the twinning rate in Denmark 1931 to 1977.Scand. J. Social Med. 8, 89.
Smith, A. (1974) Observations on the Determinants of Human Multiple Birth. Annual Report of the Registrar General for Scotland. Edinburgh.
Torgersen, J. (1951) Hereditary and environmental factors in twinning. Am. J. phys. Anthrop. 9,441.