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Thanks to the popularity of in vitro fertilization, which in its early days frequently produced multiple births, millennials are significantly more likely to be twins than any other generation. In fact, there are about a million "extra" twins among people born between 1981 and 2012.

This is more than just an interesting fact for trivia night: Twins cause and make different economic choices throughout their lives. Early on, parents of multiples are hit with higher health and living costs, as everything must be doubled. As adults, however, twins often stay geographically close to family, and are therefore more likely than other children to care for aging parents.

But perhaps the weirdest economic effect is the correlation between birth weight and lifetime earning potential: You're likely to earn more if you were a fat baby. Twins tend to both weigh and earn less. Listen to a full exploration of the effects of "Generation Twin" at NPR.