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.
It works in mice as well, they just don't have to wait a winter cycle for gestation. They basically give birth instantly after mating. They would be making _less_ babies during the low UV months and peaking during the summer time when the living is easy.
Gabriele Doblhammer and James W. Vaupel concluded that "month of birth influences adult life expectancy at ages 50+" in their paper [1] in the Proceedings of the National Academy of Sciences (PNAS).
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."
Hypothesis: people conceived from impulsive sex on Midsommar are more likely to have un-attending and less-caring parents, and possibly inherit lower self-control traits.
Avoiding the birth of a child right before winter would be advantageous for its survival… winter being (on average, not all regions/locales obviously) the hardest month to survive evolutionarily speaking this would be a beneficial trait and be likely to stick around. Ideally you want your early hominid babies at the end of winter through spring and summer so at worst they are though the first few fragile months that put the most drain on the mother, before the harshest winter weather sets in.
While not globally consistent due to regional weather differences it makes sense for there to be a broad average seasonal benefit.
Except those “early hominid” lived in Africa, which isn't known to be the place with the harshest winter. And the fact that humans living in temperate climates and humans in tropical areas belong to a single species makes me believe that adaptation to winter wasn't that big of an evolutionary factor.
Now you assume this wouldn't be something that is common for most mammals. In places where there is always sun there is no reason to evolve away people having more sex when there is sunny.
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.
I believe the phenomenon in Scandinavia that more children are born in spring is partly due to families performing economic optimization wrt. timing of kindergarten and parental leave.
-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
Doesn't it seem like a common misconception that evolution takes eons? Without being an expert, it seems plausible that we've adapted biologically since venturing out from the tropics.
The most obvious adaptation since venturing from the tropics is reduced pigmentation to adapt to less intense UV radiation. Nose shapes are also influenced by climate.
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.
But that "we" includes people that didn't venture out of the tropics, who presumably are impacted the same way by UVB, which if true would refute the hypothesis.
At a glance the study doesn't seem to investigate that at all.
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.
That's why I said "presumably". If it turns out that a group of people who've lived in the tropics for the last 50k years do not respond to UVB in the same way, then the hypothesis isn't refuted (and indeed it is validated).
> Humans are essentially tropical animals and are not equipped to deal with even mild cold. That we can live in cold climates is a result of behavioural adaptations such as wearing appropriate clothing and building shelters.
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.