Historical data reveals that having an older sibling can wreck your life

As a kid, having older siblings is either wonderful or horrible, depending on the day of the week, phase of the moon, or what stage of puberty they're going through. But over the course of your lifespan, are they generally a force for good or evil? Well, if you look at the data, it's a bit of both.

A new study published in the Proceedings of the Royal Society B used data from the huge amount of information stored by the Lutheran church in Finland for tax purposes — records of all births, marriages and deaths. Studying the 18th and 19th century, researchers were able to identify individuals with siblings, and how this relationship altered their lives.

What they found is that having an older sibling meant that you were more likely to survive to reach sexual maturity. The researchers think this may have been because older brothers and sisters were able to help support the household, either by bringing in more resources, or by directly aiding around the house. Strangely, they found that older brothers were good for both male and female younger siblings, but older sisters were just better for boys:

The fact that elder brothers had a positive effect on both younger sisters and brothers, whereas elder sisters only had a positive effect on brothers could be due to differential sex roles, which could imply either sex-specific helping behaviour, or sex-specific competition, or both. In the study population, offspring typically stayed at least until their teens in the parental household and participated in various tasks. Given boys usually worked at the farm, their participation could have increased the overall resources of the family and benefited all younger siblings independently of their sex. By contrast, girls had the opportunity to bias their help towards males as they provided direct care to their younger siblings at home.

However, once you hit adulthood, things weren't quite as rosy. They found that individuals with same-sex older siblings had lower marriage rates, fewer children, and having children later in life. The researchers attribute much of this to the rules of primogeniture, where the eldest son inherited almost everything (remember, this was data from one to two centuries ago).

What does that mean over the course of an entire life? Say the researchers:

Overall, lifetime fitness was reduced by same-sex elder siblings' presence and increased by opposite-sex elder siblings' presence.

So, blame your older siblings for your crappy life — but thank them for helping you survive childhood.

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