Published online by Cambridge University Press: 31 July 2008
Data concerning occasions of coitus were recorded by 241 married couples who had sought advice from the Catholic Marriage Advisory Council about the basal body temperature method of regulating births. The present analysis deals with the 6 days of the post-ovulatory phase which are considered to be least affected by intended birth regulation. The distribution among couples of coital frequencies is tabulated by age of wife; the mean frequency during the 6 days is 1·8 (Table 2) but this value may be influenced by the fact that some couples had avoided coitus in the pre-ovulatory phase. The hypothesis is examined that the probability of coitus on any day is unaffected by whether or not coitus occurred on the previous day. Sufficient data are available to show, as expected, that not all couples behave in the way described by this hypothesis. It is estimated that for about a third of the couples the probability of coitus is decreased if it occurred on the previous day (alternators). There are also significant numbers for whom it is increased (persisters), besides those conforming to the hypothesis.