People may say some nasty things about men marrying women half their age, but as it turns out, these men are the ones having the last laugh - evolutionarily speaking.
Statistics have shown that monogamous men have the most children if they marry women younger than themselves but how much younger is the key question.
According to a previous study of Swedish census information, a 4 to 6-year age gap is best, but new research has found that in some circumstances a surprisingly large gap - 15 years - is the best.
Martin Fieder at the University of Vienna and Susanne Huber of the University of Veterinary Medicine, also in Vienna, Austria, studied the Swedish data and found that a simple equation related the age difference of the parents to the number of offspring.
For people who had maintained monogamous relationships throughout adulthood, the most children were found in couples where the man was 4.0 to 5.9 years older than the woman. The probable reasons behind this state of affairs are not controversial:
"Men want women younger than themselves because they are physically attractive, while women tend to prioritize a partner who can provide security and stability, and so tend to opt for older men," New Scientist quoted Fieder, as saying.
Erik Lindqvist at the Research Institute of Industrial Economics in Stockholm, Sweden, pointed out that the age of the mother is likely to be more important than any age difference: the older the mother, the lower her chances of having more children.
"We added that factor into the calculation," said statistician Fred Bookstein at the University of Washington, a colleague of Fieder and Huber.
"The importance of the age difference didn't change," Bookstein added.
The study is published in Biology Letters.