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

1. 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)

2. 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

3. 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

4. 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

5. 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

6. 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

7. Adimensionalization
rescaling

8. The map x0 → x∞
numerically

9. Total sexual activity
frequency of intercourse
time

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