Frequency of sexual intercourse in permanent couples Sergio Rinaldi DEI, Politecnico di Milano, Milano, Italy EEP, IIASA, Laxenburg, Austria

The first model in sexology Summary The first model in sexology A model is simply a set of conjectures (axioms) A new model can be identical to an already existing model (conceived in another field) A model can have infinite attractors and repellers A model can have no bifurcations (universal model)

There are rich data sets (see references) Empirical studies There are rich data sets (see references) From statistical analyses of data it is possible to conclude frequency of intercourses time Tmax

Martin’s law (Journal of Sex Research, 1970) An old folk theory Martin’s law (Journal of Sex Research, 1970) first year of marriage after first year of marriage At the end of your life there are still beans in the box

Martin’s law (Journal of Sex Research, 1970) An old folk theory Martin’s law (Journal of Sex Research, 1970) frequency of intercourse time Tmax first year of marriage after first year of marriage

ODE model A conceptual model : erotic potential at time : sexual appetite at time : frequency of sexual intercourse First axiom Second axiom Third axiom ODE model

A conceptual model : erotic potential at time : frequency of sexual intercourse The proposed ODE model is identical to the first epidemic model proposed in 1925 (Kermack and McKendrick) infinite attractors and repellers

Adimensionalization rescaling

The map x0 → x∞ numerically

Total sexual activity frequency of intercourse time

Martin’s law Conclusion: Martin’s law is wrong (but our model overestimates NII ...)