If we really want to try to take that seriously, I would add several things to the model: * trend for increasing life expectancy and average reproduction age (

If we really want to try to take that seriously, I would add several things to the model: * trend for increasing life expectancy and average reproduction age (I am not saying age of menopause - there are probably pretty hard biological limits on that, but women increasingly choose yo have babies later in life). * accounting for reproduction rate being dependent on overall population and/or probability density in a given region * Migration from high reproduction regions to rich low reproduction regions