AN HYPOTHESIS ON THE ASSOCIATION BETWEEN MATERNAL SMOKING AND DIZYGOTIC TWINNING

There is good evidence that smoking is a marker for high steroid hormo ne concentrations (at least at the time that smoking is initiated). Th is 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 u se of oral contraception) are markers or indices of high hormone conce ntrations may have widespread repercussions for cross-sectional epidem iological studies of such risk factors for diseases thought to be part ially caused by high (e.g. prostatic and breast cancers) or low (e.g. rheumatoid arthritis) concentrations of these hormones.