Coital frequency and twinning. A comment
James, W.H.
Journal of Biosocial Science 24(1): 135-136
1992
ISSN/ISBN: 0021-9320 PMID: 1737810 Document Number: 317336
The author comments on Bonnelykke et al's 1990 article on the coital rates of 390 mother of dizygotic twins (DZ), and finds that the data do not support the hypothesis that coital rates of mothers of DZ are higher than those of controls. The coital rates were 9.0 and 9.3/month respectively. Questions are raised as to the strength of the evidence, the sagacity of abandoning the hypothesis, or the probability of this sample size detecting differences. Cohen in 1977 hypothesized mean coital rates of DZ and control mothers and James in 1984 estimated rates of 16 and 15/month respectively. The coefficient of variation or the standard deviation divided by the mean has been found to be 2/3 regardless of the mean when age, parity and social class are controlled for. The harmonic mean of Bonnelykke's 2 sample sizes of 390 and 1206 is 589. Using Cohens Table 2.3.2 at 600 and at d=.1, the probability of detecting the hypothesized difference is .53 at the .05 level, which is impressive but not decisive evidence. Biological evidence supports the proposition of high coital rates at the time of conception and the birth of DZ twins. In double ovulation, oocytes are released usually at different times. The corpus luteum will have an inhibitory effect on the 2nd fertilization, but this condition must occur at 1st fertilization instantaneously. A plausible situation is the 1st fertilization is with an aging spermatozoa and the 2nd from a fresh ejaculate. Also supporting this notion is the fact of superfecundation, where fertilization occurs from spermatozoa from different ejaculates.