An analysis of the records revealed that men who had partners aged six years younger than themselves had the greatest number of children: about 2.2 children on average, a relatively high number in a country that has long had low fertility rates in the past.
By comparison, men who had partners six years older than themselves had 1.8 children on average.
Women bore the greatest number of children when they chose men four years older than themselves. Those who partnered with such men had 2.1 children on average.
The age-gap figures and numbers of children do not match up because the people studied were randomly selected from a large number of the Swedish population and therefore not likely to have been partners with each other, explain the researchers.
When too much of an age difference exists, there is perhaps a greater chance of male fertility problems and disagreement over family planning, suggests Fieder.