Could be a signaling behavior to encourage spring births. Does UVB peak in the middle of the summer? 9 months later is probably the best time to birth a child for that child's success.
They tracked for 30 years the mortality of all Danes who were at least 50 years old on 1 April 1968 (about 1.4 million people born before April 1918). They also analyzed about Austrians with known birth dates who died between 1988 and 1996 (about 700,000 people born before 1947) and native-born Australians who died between 1993 and 1997 (about 200,000 people born before 1948).
They found in Denmark and Austria that "adults born in autumn (October–December) live longer than those born in spring (April–June). The difference in lifespan between the spring and autumn born is twice as large in Austria (0.6 years) as in Denmark (0.3 years). ... We found the pattern in the Southern Hemisphere to be a mirror image reversal of that in the Northern Hemisphere." British born Australians were statistically closer to the Danes and Austrians.
Their analysis eliminated three hypotheses for these observations: seasonal distribution of deaths, social factors related to seasonal distribution of births, and differential infant survival. Their analysis and other studies of birth weight data led them to conclude that "seasonal differences in nutrition and disease environment early in life [in utero and infancy] could explain the relationship between month of birth and adult lifespan." Over the years, winter and spring nutrition has improved, so "the relationship between month of birth and lifespan seems to be stronger among the older birth cohorts than among the more recently born."
While not globally consistent due to regional weather differences it makes sense for there to be a broad average seasonal benefit.
Meanwhile here in northern Europe we get markedly more children during March-April. Which of course could be a sociological thing, or depend on something else, but still.
-out of pocket maximum expenses are capped per calendar year. Typically, healthcare expenses for births happen in the latter half than former, and so you can potentially save a few thousand dollars, and even up to $17.4k (legal max). Post birth complications are plenty, and pelvic floor physical therapy does wonders. Not worrying about costs for these is a huge benefit.
-new resident doctors start in June, so you have less risk of a brand new doctor in April/May when they would have had 10 months of experience
-kid would be 6 months of age by winter season, so eligible for flu vaccine, and now Covid vaccine too during their first winter
-lots of schools have hard cutoffs at Aug 31 or around there. Birthdays before then, kid is in school, and after that, kid has to wait another year to start school. This could mean 6+ months of fewer daycare expenses, easily worth $10k
So this is why I hate winters and don't complain when it's 90 degrees Fahrenheit outside.
I would wear a jacket or hoodie in the middle of summer because of the office AC.
I need to move to Key West.
The authors are from Israel, so I guess the participants are Israelis too. That's one of the more ethnically diverse places, but only 12% of Israelis are of African origin. Of course not all of Africa is in the tropics, but there are a couple of Asian Israelis who are from the tropics, so let's estimate it at about 12% with mostly tropical ancestors. That might just vanish within the noise of the data unless you specifically look at the correlation to ethnicity.
The idea that we stopped evolving sometime before we spread over the globe is obviously false, with the number of adaptations that are plainly observable on the outside.