Smarter Siblings and Sturdy Immune Systems

A new wave of studies suggests that a person’s position among siblings has a lasting impact on their life – older siblings are more likely to exhibit higher intelligence and younger siblings more likely to be healthier. Which sibling are you? The older and more knowing one, or the younger, more confident and rebellious sibling?

In 2015, over 20,000 British, American and German citizens conducted a battery of personality tests in an observational study led by Dr Schmuckle at the University of Leipzig. They concluded personality, including emotional stability, agreeableness and imagination, is not affected by the position we have in a sibling group, contrary to the long-standing stereotype. Despite this, they exhibited slight intelligence differences – older children tended to be smarter than their younger siblings. First-born children were more likely to agree they are “quicker to understand things” than younger siblings. They had richer vocabulary and outdid younger siblings on abstract understanding problems.

The underlying biological cause for this difference in intelligence is still unclear, however some speculate higher social rank plays a functional role.

Still, being a younger sibling is not all that bad. Older siblings now have reason to envy younger family members – they are actually healthier! In a group of similar observational studies, one led by a team at Queen’s University Belfast, sibling’s born first exhibited a higher risk of developing Type 1 diabetes than those born second, third or later. It has been suggested older siblings attending school bring home bugs that stimulate the younger siblings’ immune systems – reducing the risk of it incorrectly attacking the body in a way that could give rise to autoimmune disease like Type 1 diabetes. Other neonatal or postnatal factors could also have an effect, for example a delayed exposure to infections could result in weaker immune system in older siblings. Again, these are just speculations and the exact cause is unclear.

It is important to remember that complex traits such as intelligence, or the strength of an immune response are determined by a multitude of factors – genetics, environmental exposure, mother’s age at birth, family size. We are a long way from understanding their complex interconnections and even larger studies need to be carried out to underpin the subtle differences and factor effects.