Mick Jagger, Rupert Murdoch and Michael Douglas all have the right idea, evolutionarily speaking. Statistics show that monogamous men have the most children if they marry women younger than themselves. How much younger is the key question.
Last year, a study of Swedish census information suggested 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 optimum.
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," says Fieder, while women tend to prioritise a partner who can provide security and stability, and so tend to opt for older men.
Mum's the Word
However, Fieder and Huber's calculations drew criticism. For example, 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," says statistician Fred Bookstein at the University of Washington, a colleague of Fieder and Huber. "The importance of the age difference didn't change."
Even if it holds true for Sweden, the 4 to 6-year age gap is unlikely to be optimal in all cultures. Samuli Helle at the University of Turku in Finland read Fieder and Huber's paper and says it stirred memories of an unpublished study he conducted a few years ago.
"In 2001, I studied the demographics of the Sami people of northern Finland," he says. "I had thought I had missed the opportunity to publish, but when I saw the Fieder and Huber paper I thought: why not write a response?"
Helle's team performed a similar calculation to Fieder and Huber's, using the demographic data from the 17th to 19th centuries that Helle had already collected from northern Finland. For the Sami people, they found that males with 15 years on their partners had the most children.
"I don't know why the optimal age differences were so much bigger among the Sami people, but it might be related to culture," says Helle, noting that the Sami were nomadic reindeer hunters. "Perhaps those huge lifestyle differences are important."