Setting is a dark ages medieval world. Equivalent to Earth circa century V AD.

Agriculture has no crop rotation, and most of the harvest is lost to plagues and pests (those damn wabbits).

Would children born in winter become weaker people as they develop than the children born in other seasons?

If they are not with just the premises above, what other elements should I add to achieve this goal?

I'm not only referring to babies. It should be something that can hinder development in all ages (i.e. they become weaker adults too).

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    $\begingroup$ They won't be weaker due to harsher conditions. They will be stronger, due to natural selection. $\endgroup$ – The Square-Cube Law Jul 31 '18 at 17:56
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    $\begingroup$ I disagree with @Renan. Take a look at people moving from poor countries. You will often see the huge child that was born in this country, with his tiny parents or grandparents that grew up in poverty $\endgroup$ – Andrey Jul 31 '18 at 18:12
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    $\begingroup$ @Andrey exactly. Europeans and "First World" Asians (from places like Japan, Korea and Taiwan) born in the past 70 years are significantly taller than their predecessors. $\endgroup$ – RonJohn Jul 31 '18 at 20:07
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    $\begingroup$ ... Depends on how starved their mum is for the first year/few years... and I suppose how well sheltered the kid would be from the elements. Most pre-modern societies I believe (Not an expert here) breastfed for a few years rather than only a few months, which would level the playing field more - As summer kids wouldn't be weaned in winter and vice versa. $\endgroup$ – Hannah Jul 31 '18 at 20:40
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    $\begingroup$ @Andrey: That's a reasonable theory, one that's not hard to test. Which people have done, and they've concluded that the Zodiac is still full of crap. $\endgroup$ – BlueRaja - Danny Pflughoeft Jul 31 '18 at 23:41

13 Answers 13


Tuberculosis likes the winter.

sanatorium, 1925

Tuberculosis was a leading cause of death up until this century. In areas with endemic TB, infants and children are at great risk for new infection and with their lower immunity (as well as immunity defects from malnutrition) their risk of death is higher. The risk of contracting TB as an infant or child is higher in the winter.

Seasonality of Tuberculosis

The observation of seasonality leads to assume that the risk of transmission of M. tuberculosis does appear to be the greatest during winter months. Vitamin D level variability, indoor activities, seasonal change in immune function, and delays in the diagnosis and treatment of tuberculosis are potential stimuli of seasonal tuberculosis disease. Additionally, seasonal variation in food availability and food intake, age, and sex are important factors which can play a role in the tuberculosis notification variability.

If you goal is for the winter-born to grow up as weaker people, being consumptive will accomplish that. Consumptives were frail, anemic, easily winded, and unable to participate in rough-and-tumble activities of childhood. Tuberculosis can flare up and settle back down, following a remitting and relapsing course. It can stunt growth and if these persons with TB were lucky enough to survive to adulthood they would be small and frail.

consumptive girl https://www.pbs.org/wgbh/americanexperience/features/plague-gallery/


Here's a fun, unforeseen effect!

Did you know that, in England, you're considerably more likely to be a professional athlete if you're born between September and December?. That same article continues:

This is not a one off. At a recent U17 European Championship, 75% of the footballers were born in a four month window. Similar results were detected in Canadian ice hockey and at the 2008 Beijing Olympics.

In fact, if you look across all sports at all levels, you'll find a disproportionately large representation of people born within a few months. That seems to imply that there's a "best" time to be born if you want to be a professional athlete.

However, what's important here isn't really the month a person is born; it's the month that sport's season starts.

This is called the Relative Age Effect, and it actually affects all walks of life with a routine "season". Lets change gears for a second and look at Wikipedia's example. They state that most soccer leagues "use 1 January as their administrative cut-off date when determining an athlete's eligibility to compete in youth competitions, children born before a specified cut-off date are excluded."

So, lets consider a child born on January 2nd; we'll call him Jim. Let's also consider a child born on December 31st of the same year, and we'll call him Dan. Now, Jim and Dan both love soccer. At the age of 6, they both sign up, and since they're both born after the latest cutoff, they're put into the same year. Except, as Wikipedia puts it, "a six-year old born in January is almost 17% older than a six-year old born in December in the same year." So not only is Jim older than Dan, he's probably much larger.

So as the season starts, Jim is going to stand out considerably more; for all intents and purposes, he's a year older than Dan. Then, when the second season rolls around, Jim's still a "year" older, but now his coach picks him to be captain because he stuck out so much last year. Then the next year he's picked again, and so on. Dan never really has a chance here, beyond being an exceptional athlete.

How does this apply to my question?

Well, you've asked if children being born a certain month might be weaker, to which I'll ask: is there a routine event every year that kids participate in? Unfortunately, the harvest doesn't really line up with this idea. If anything, it suggests the opposite; kids born in the winter are born early in the harvest "selection process". When all the 6 year old make it out to the fields, your winter babies are going to outperform the rest.

But maybe there's other structure in your world. Maybe the serf mud-wrestling season starts on January 1. Maybe that's when school starts (for those peasants lucky enough to go). These would give a disadvantage to the September-December babies.

In other words, if you want winter babies to have a developmental disadvantage throughout life, have a developmentally significant routine in their lives start right after winter.

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    $\begingroup$ the routine itself isn't important but where the age-cutoff date is. $\endgroup$ – ratchet freak Aug 1 '18 at 8:29
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    $\begingroup$ The same is true academically, between august and september babies. $\endgroup$ – WendyG Aug 1 '18 at 12:12

There would probably be a correlation, I base that on studies on people born during medieval famines and their long term development, it may not hold on the shorter seasonal scale though. Winter is not usually the hungriest season in a subsistence farmers year however, it would be the children born in early to mid-spring that I would be most concerned about. Spring is a busy time on the farm, planting the years crops etc... and there's very little coming in with the main crops just going in the ground and most of the winter's stores depleted. Summer and Autumn are the best seasons for the food/labour balance in agrarian societies.


I think your hypothesis would be true if pregnancy and rise would be enclosed in a couple of seasons, like it happens for those wabbits or other animals.

Humans, or better women, take 9 months to deliver a baby. This means that if they deliver in winter they have been pregnant during months with relative abundance of food. Thus the baby might have received a better nourishment while in the womb. The opposite can apply to babies born in summer: scarce nourishment in the womb, but better intake during early grow up phase. All in all I think the compensate each other.

Also, considering that development takes year, it's not a few months of scarce food which can severely hamper development. Different story if the scarcity is spread over several years.

To support my statement, consider that humans have not developed a seasonal reproduction, but can do it all year round. Since abundance of food has been a recent achievement for most of mankind, if such an influence was present during our evolution it would have brought to some visible consequences.

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    $\begingroup$ The baby receives most weight (about 70%) in the last three months of the pregnancy. Weight is an important factor for survival after birth because the baby will need the reserves, especially in the first month. If the last three months are at late winter and very early spring, the weight should be considerably lower and making the babys weaker. $\endgroup$ – Mixxiphoid Aug 1 '18 at 6:14

There is one thing that nearly all children born in winter suffer from more than those born in sunnier seasons:
Vitamin D deficiency.
Vitamin D deficiency in early childhood leads to Rickets, a sickness where the bones are weaker and deformed. Supplementation with Vitamin D can reverse this in young children, older kids will be left with skeletal deformities for life (2).
The results of this is that the affected persons, depending on the severity of the Vitamin D deficiency, would

  • be unable to walk long distances when older (duo to bowed legs and knock knees, which put greater stress on the cartilages)
  • have an affinity for lung diseases (duo to – not necessarily visible – deformities of the breastbone)

The first issue impacts daily life greatly as it makes activities with a lot of walking and carrying painful. So professions such as soldier, farmer or builder would become harder for those afflicted, especially as they progress in age. The second issue is possibly deadly, particularly in a place without antibiotics.

Mind that these deformities are not disfiguring, they may well fall into the range of what occurs normally.

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The effect of malnutrition during pregnancy and early childhood has been researched in cohorts born during the Dutch hunger winter and the Great Leap Forward.

To quote a survey of the now quite extensive literature: "The studies show consistent associations between prenatal famine and adult body size, diabetes, and schizophrenia. For other measures of adult health, findings are less robust."

Notably neither height nor IQ seems to be affected in any significant way. I think it is save to say that seasonal malnutrition wouldn't lead to any noticeable differences between people. Even the known correlations wouldn't lead to differences visible without a statistical analysis.


A child born in the winter wouldn't be weaker unless conditions were so severe that it impacted every day survival like access to food and human interaction over prolonged periods of time.

For example any child born into conditions where long-term famine exists will have some physical/mental delays. If a mother doesn't have access to nutrition her milk supply would be affected if she couldn't supplement that with some external food source (they ate the cow/goat/alternate milk source) the baby would be in trouble. Is the child sucking on a wet rag to calm hunger pangs? Is it getting food it can't digest? The level of malnutrition and how its sated will be relevant.

Note: this would also affect potential antibodies gained from the mother.

Beyond the physical needs not being met parents who are scrounging for food/warmth in general don't have the time (or energy) to interact with a child which would impact verbal/critical thinking skills. There is a strong correlation between how much you interact with a child and how readily they learn language and eventually other skills. That isn't to say that the children couldn't catch up... that depends on how long they were deprived and how circumstances have effected the child since then.

There is also some very interesting information on how things like famine imprint in your genetics and affect your offspring generations away. Maybe something in that research could help you out. Google "genetic implications of exposure to famine"

However, under normal conditions people seem to thrive just fine in the cold (think Alaska)

  • $\begingroup$ Well Done and Welcome to Worldbuilding. quick hint: Add some links instead of just saying "google such and such". The search suggestion is great, but a link helps the Questioner and other readers get to the meat of the matter more quickly. Have Fun $\endgroup$ – Paul TIKI Jul 31 '18 at 16:25

I'm going to answer this as if it was a biology.SE question, since it is an interesting scientific question (what is the impact of season of birth on long term development) and asks for a science-based rationale.

There is good evidence for a long term effect of prenatal nutrition on immune system function in humans, correlated with season of birth. To quote the results section of the abstract:

Adult death was associated with a profound bias in month of birth with 49 cases born in the nutritionally-debilitating hungry season (Jul-Dec) versus 12 in the harvest season (Jan-Jun). Relative to harvest season the hazard ratio for early death in hungry-season births rose from 3.7 (for deaths >14.5 years, P = 0.000013) to 10.3 (for deaths >25 years, P = 0.00002). Anthropometric and haematological status at 18 months of age was identical in cases and controls, indicating an earlier origin to the defect.

There are plenty of other relationships between prenatal or early environmental insults that cause long term adverse health effects, and some other interesting data on the relationship (in human data) and a proposed mechanism (based on an animal data) for birth month and psychiatric diseases but this one is particularly striking.

As an aside, the answers and commenters suggesting that individuals who survive an environmental challenge are necessarily stronger because of natural selection have misunderstood a few basic principles. In natural selection, fitness is about passing on genetic material, not about being stronger. Nothing about natural selection suggests that survival in one condition (e.g., the winter of your early days) makes an individual necessarily better suited to another environment in another developmental stage.

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    $\begingroup$ n=61? Also, almost all positive traits are correlated, possibly due to the influence of genetic load, therefore survival in one condition does imply being better suited to other environments. $\endgroup$ – BlindKungFuMaster Aug 1 '18 at 7:09
  • $\begingroup$ @BlindKungFuMaster n=61 is a perfectly valid sample size here, as you can see from the analysis. Re: "almost all positive traits are correlated", that is kind of a nonsensical statement. You can make a straw man argument around some specific cases , but it's not generally applicable. A positive trait is by definition dependent on the environment. Consider Darwin's finches. A large beak is a positive trait in one environment, a negative trait in another. $\endgroup$ – De Novo Feb 19 '19 at 6:32

More likely is that children nursed in the winter would be weaker, since the mother has less food.

But since pregnancy lasts nine months, and nursing last for 12ish months, and those pesky wabbits eat your crops in the summer, your poor child has development problems all year long.

Which is why all people (especially peasants) were so short back then.

  • $\begingroup$ Traditionally cultures tended to have kids fully weaned by 2-3 years old, not by the end of the first year like in modern societies. Which kinda levels the playing field in this respect. $\endgroup$ – Hannah Jul 31 '18 at 20:50
  • $\begingroup$ @Hannah Many of my farming ancestors had children much more frequently than every 3-4 years, so I'd like to see any information you have on which and when cultures weaned after 2 years. (I can't imagine patriarchists waiting that long to, well, you know.) $\endgroup$ – RonJohn Jul 31 '18 at 22:10
  • $\begingroup$ @RonJohn: Breast feeding is a natural contraceptive. $\endgroup$ – BlindKungFuMaster Aug 1 '18 at 7:15
  • $\begingroup$ @BlindKungFuMaster tell my ex-wife, who got pregnant at 8 months while she was still breastfeeding. $\endgroup$ – RonJohn Aug 1 '18 at 13:01
  • $\begingroup$ ... @RonJohn you can breastfeed more than one child at once. $\endgroup$ – Hannah Aug 3 '18 at 10:07

The easiest way to ensure that children born in the winter are significantly worse off than children born in other seasons is to have some condition in the spring that makes conceptions at that time different.

For example, you could have some kind of religious fasting period at a critical stage of development: if your world had something like Lent where everyone past adolescence gives up most of the foods that contain folic acid for a month or two it probably wouldn't have a terrible effect on the adults (who could make that up in the other months of the year) but it could be devastating for babies conceived when their mothers are folate-deficient. Other adverse gestational conditions either at the time of conception or at some point during pregnancy might also contribute to poor long-term outcomes, though you'd probably want to pile them up to ensure a noticeable effect.

Or perhaps you have a tradition that leads to the conception and eventually birth of many babies who are socially disadvantaged. The obvious option would be some kind of widespread, state-sanctified rape during the spring months, so that a large portion of children born nine months later have no paternal support and may have mothers who are ambivalent-at-best about their children. These children would be materially disadvantaged, publicly stigmatized and, in many cases, would have much more emotionally precarious relationships with their mothers compared to children born from non-violent unions. All three factors—poverty, social ostracism, and poor attachment to a primary caretaker—are likely to lead to poorer long-term outcomes.

Finally, you could create a self-fulfilling prophecy: it is known to be bad luck to be born in the winter, so only those who are too poor (if anything like effective prophylactics exist), too lacking in self-control or a partner who cares enough to exercise self-control, (see above for the possible effects of the latter) or too ignorant to prevent conception are likely to get pregnant then; the children who are accidentally conceived in the "bad" months are therefore mostly born into poverty and without the advantages of education, both of which will statistically lead to worse outcomes.

And, in fact, there are some measurable differences in various outcomes for infants born in the US depending on birth month, and a 2013 study suggests that something like this last mechanism may play an important role (emphasis added):

Research throughout the social and natural sciences has demonstrated an association between the month of a child’s birth and a variety of later outcomes, including health, education, and earnings. Past explanations of this relationship have been limited to factors that intervene after conception, such as compulsory schooling laws or seasonal exposure to disease and nutrition. In this paper, we consider the possibility that individuals born at different times of year are born to mothers with significantly different characteristics. Using birth certificate data and census data, we document large and regular seasonal changes in the socioeconomic characteristics of women giving birth. Women giving birth in winter are more likely to be teenagers and less likely to be married or to have a high school degree. These effects are large in magnitude and are observable for children born throughout the second half of the twentieth century. We show that these seasonal changes can account for a large portion of the poorly understood relationship between season of birth and other outcomes.
Buckles, Kasey S., and Daniel M. Hungerman. “Season of Birth and Later Outcomes: Old Questions, New Answers.” The review of economics and statistics 95.3 (2013): 711–724. PMC. Web. 2 Aug. 2018.

The likely mechanism identified for this difference is that women who plan their conceptions seem to be actively avoiding winter weather (based on historical average weather conditions in the expected, rather than actual, birth month by country of delivery).


Actually I think you might want to consider turning this on its head. Assume that in tougher conditions, the weak die off and only the strong survive.

The general fortitude and development of the child is heavily predicated in it's genes, a dice that is rolled long before birth. The availability of water, nourishment and ideal living conditions sustains the growing child.

In ideal conditions even the "runt of the litter" has a decent chance of reaching maturity, and hence your summer children would be more likely to feature physically weaker and less developed specimens than their winter-born cousins


I think that you should probably add a "tournament" or a "challenge" for the kids of a natural year, I mean, each 2 Aprils for example all the kids do some competition against all the other kids from his year till they are 8 years old and making a kind of ranking, this to determine some future "jobs" or positions in the society, making the kids bornt in the last months of the year weaker because they would be smaller and younger. With this you will get weaker adults too cause they will be in worst society positions and they will have less food.

I am actually trying to give something different from "they won't have enough food" "they will starve" and this type of answers, although I really like the "tuberculosis answer" from Willk


I would say that even if the children that were born in winter were weaker, then the surviving ones would actually become stronger adults (on average) as the weaker would die. But I would not expect big variations as a child is in the womb for circa 9 months and then is really fragile for a year or more. So the weak ones will die regardless of the time they were born.

You would see at the Middle Ages when the life expectancy of the whole population was about 30 years, but if you take only those that survive childhood (i think it was from age of 10 and above) then the life expectancy improved significantly.

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    $\begingroup$ The average lifespan was ~30yo. When 1/3 of you lives to 60 (to pick a number), 1/3 dies in childbirth, and 1/3 of whoever is left over lives for somewhere between 1 and 59 years, then the average is 30. $\endgroup$ – Mazura Aug 1 '18 at 1:08
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    $\begingroup$ @Mazura also women died a lot more during labor, and the average age of a women giving birth (and die) was certainly below 30. Skewing the graph even more. $\endgroup$ – Mixxiphoid Aug 1 '18 at 6:18
  • $\begingroup$ @Mixxiphoid most women did not die in childbirth. Nowadays, the highest maternal death rates in poor African countries have about one in sixteen women die of pregnancy related causes, and in medieval Europe that may have been lower, because they married relatively late and had fewer pregnancies. $\endgroup$ – pidan_dan Aug 2 '18 at 7:08

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