An hypothesis on the association between maternal smoking and dizygotic twinning

There is good evidence that smoking is a marker for high steroid hormone concentrations (at least at the time that smoking is initiated). This would explain the finding that smoking is associated with dizygotic, though not monozygotic, twinning. The notion that smoking and other elective behaviours (e.g. drinking alcohol, opting for vasectomy and use of oral contraception) are markers or indices of high hormone concentrations may have widespread repercussions for cross-sectional epidemiological studies of such risk factors for diseases thought to be partially caused by high (e.g. prostatic and breast cancers) or low (e.g. rheumatoid arthritis) concentrations of these hormones.