11. Many thanks to everyone in the Dugdale research group and everyone else who helped with this project, but especially to my amazing supervising team!

10. Overall, our study highlights the importance of taking into account the sex and age of older siblings when examining their effect. We further show in this preindustrial population how biology and culture both affect how older siblings affect the survival of their younger siblings.

10. Finally, although the effect was not significant, we also find some evidence of beneficial effects of older brothers far in age on childhood survival. This effect can be driven by for example their economic contributions to the family. Older sisters often received less pay for the same work.

9. We further find a positive effect of older sisters close in age on survival of their siblings. While this could be due to providing care it seems unlikely (because we find no effect of older sisters far in age). Perhaps these individuals were less likely to suffer due to a lack of older brothers.

8. This negative effect can be possibly due to son preference present in the population resulting in a biased investment of parents towards son. This can also be due to the biological need for more resources. Either way younger sisters are at detriment when an older brother close in age is present.

7. What we find is that the number of older siblings in general does not affect childhood survival. However, upon dissection we see effects of specific categories of older siblings appear. Most notably the negative effect of older brothers close in age on only their younger sisters' survival.

6. We use historical individual life-history records spanning from 1750-1870 of almost 3000 Swiss individuals (Glarus), and examined how having more or less older siblings affected childhood survival. We especially focused on if older siblings died, were close or far in age, and which sex they were.

5. Sex can complicate sibling interactions further. Boys are assumed to be more costly and can be favored in populations with son preference resulting in more competition. Sex can also determine how siblings help, sisters are often providing more direct care while brothers might provide resources.

4. Age can influence whether older siblings cooperate or compete with their siblings. Older siblings close in age are more likely to compete for similar resources and are unable to provide care or resources making them more likely to compete. While the opposite can be said for those further in age.

3. However, older siblings can also act cooperatively providing care for their younger siblings or by providing resources. These counteracting roles of older siblings can have as a result that many publications report different findings; from purely negative effects to solely positive effects.

2. Older siblings play a large role especially in the early life of younger siblings. One idea is that there is a tradeoff between reproduction and parental investment. Where having more siblings results in less investment per child. Thus we can expect siblings to be competitors for these resources