The monozygotic twinning rate: is it really constant?
Weinberg's difference method, applied to twin birth statistics, usually shows a dependence of the MZ rate on maternal age, like a thin shadow of the DZ rate. Some of this MZ variation could be explained away by James' finding of more same-sex (SS) than opposite-sex (OS) DZ twins, the excess being mistakenly classified as MZ by Weinberg's assumption of equal numbers. By several methods one can extract a constant value for the MZ rate and a constant or nearly constant value for the DZ SS/OS ratio, but these "constants" are actually arbitrary and they vary between populations.
Allen
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U.S. Public Health Service, Bethesda, Maryland.
Hrubec
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D001722:Birth Order; D001723:Birth Rate; D005260:Female; D006801:Humans; D008297:Male; D008423:Maternal Age; D044469:Racial Groups; D012744:Sex Ratio; D013223:Statistics as Topic; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic