This makes no evolutionary sense to me…A new study claims 15 years age difference between man and a woman was an ideal difference for maximizing their biological fitness (number of surviving children, therefore genetic output).

The study looks at 17-19th century Sami people from Finland which were under influence of natural and sexual selection as natural fertility and mortality rates applied, argue the authors.
However, further back in our history we must have been even more at the influences of nature.

Assuming that people lived shorter in our evolutionary history than they do now and that women reached puberty and sexual maturity later than they do now, to achieve this age difference man would have to be around 30 years old in order to marry a reproductively mature woman who is 15 years younger than him.

Now is it just me or would this be some 15 years of reproductive potential wasted for this man? I find it difficult to believe that benefits of him being so much older would be enough to outweigh the costs of not reproducing for such a long time. We have to assume that such an older man would have the better resources to support children, but wouldn’t life span be much shorter in those days and wouldn’t 30 year olds be quite old in those days?

Image of Sami people from around 1900: