On average, the lifespan of children born in years that had a great deal of solar activity was 5.2 years shorter than other children.
The largest difference was in the probability of dying during the first two years of life.
“There are probably many factors that come into play but we have measured a long-term effect over generations. The conclusion is that you should not sunbathe if you are pregnant and want to have a lot of grandchildren,” said lead researcher Gine Roll Skjaervoe from the Norwegian University of Science and Technology’s (NTNU) department of biology.
The team also found that children who were born in years with lots of sunshine and who survived were also more likely to have fewer children, who, in turn, gave birth to fewer children than others.
The finding shows that increased ultraviolet (UV) radiation during years of high solar activity had an effect across generations.
For the findings, Skjaervoe studied church records from the period 1750-1900 involving 9,000 people.
Skjaervoe used information on the number of sunspots as an indication of the amount of UV radiation in a given year.
The number of sunspots reaches a maximum every 11 years on average, which results in more UV radiation on Earth during years with high sunspot and solar activity.
UV radiation can have positive effects on human vitamin D levels but it can also result in a reduction of vitamin B9 (folate).
It is known that low folate levels during pregnancy are linked to higher child mortality.
The study also showed that families from the lowest socio-economic groups were most affected by UV radiation.
Women who worked in the fields were more exposed to the sun than other women.
In many cases they also had a poorer diet.
The study appeared in the journal Proceedings of the Royal Society B.