We finally know why oldest children do so much better than their younger siblings

My husband and I have three children. The oldest gets most of our time and attention — and that’s normal, according to a landmark new study released this week that tracked 1.2 million people over 40 years.

Oldest children also have a measurably better shot at higher incomes, better education and better mental health than youngest children. And for the first time, we have an idea of why.

Birth-order effects have been documented for the last 20 years. A major study in 2005 found that younger siblings fare worse than firstborns on lifetime earnings, educational attainment, mental health and, for women, teen pregnancies.

In fact, the further back in birth-order you fall, the worse the numbers get. This is a very scary and grim concept for parents, and no one had any idea why … until now.

The new study, which will be coming out in the American Economic Review, followed 1.2 million Danish children from birth to adulthood using government records — an incredible dataset. They found two very striking reasons for the birth-order gap, and they make sense intuitively.

Exposure to respiratory viruses in the first year of life, before the immune system is mature and before most childhood vaccines kick in, consistently predicted lower earnings, lower educational attainment and worse mental health decades later. The mechanism by which the younger sibling gets said respiratory........

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