What Are the Odds of Twins Born in Different Years?

The following is a guest post by Louise Firth Campbell and Amram Shapiro, the authors (with Rosalind Wright) of The Book of Odds: From Lightning Strikes to Love at First Sight, the Odds of Everyday Life.

Rare news stories recur with surprising regularity -- one of these annual stories is the birth of twins in two different years. 

You can see the appeal to editors. Babies are photogenic, especially twins. The symbolism of the New Year as a baby which ages to dotage by year end is an old one, a staple of thousands of New Year’s cards. There is an interesting apparent time tension in the story. Twins share a womb and genomes. Yet a few seconds separation in time of birth makes an apparent year’s worth of difference.[1]  This event is considered a rarity and only a handful of cases are reported in the press each year. This rarity makes it news, but is the event really as rare as it seems?

Let’s start with the news in 2013/2014. If the cases reported in the North American media, two in the U.S. and one in Canada, are the only cases, these events are rare indeed. There are about 4 million births a year in the U.S. That would suggest these events are as rare as 1 in 2 million. Someone visiting the Grand Canyon is more likely to die by falling off the edge (1 in 1.5 million).[2]  That doesn't feel right to us.

“Gayborhoods” and Heat Waves at This Year’s AEA Meetings

Jon Hilsenrath of The Wall Street Journal reports on the most offbeat papers of this year's American Economic Association meetings.   One of our favorites -- in light of our recent "Are Gay Men Really Rich?" podcast -- is this one:

FIND A NEW “GAYBORHOOD” FOR BETTER HOUSING RETURNS

Janice Madden of the University of Pennsylvania and Matthew Ruther of the University of Colorado studied census tract data and the American Community Survey to examine the locations of gay male and lesbian partnerships in 38 large U.S. cities. They found that census tracts that start the decade with more gay men experienced significantly greater growth in household incomes and, in the Northeast and West, also greater population growth over the next decade than those census tracts with fewer gay men. Census tracts with more lesbians at the start of the decade saw no difference in population or income growth.

Another favorite examines the long-term outcomes of children conceived during heat waves.

Different Kinds of Moms Have Babies at Different Times of Year

We've written in the past about the relationship between a child's month of birth and a variety of later outcomes. In SuperFreakonomics, for instance, we wrote about research by Douglas Almond and Bhashkar Mazumder showing that "prenatal exposure to Ramadan results in lower birth weight." In a Times column called "A Star Is Made," we examined the link between birth month and accomplishment in sports. We also noted, however, in SuperFreak, that the sports advantage -- and probably many other birth-month influences -- are relatively small:

But as prevalent as birth effects are, it would be wrong to overemphasize their pull. Birth timing may push a marginal child over the edge, but other forces are far, far more powerful. If you want your child to play Major League Baseball, the most important thing you can do — infinitely more important than timing an August delivery date — is make sure the baby isn’t born with two X chromosomes. Now that you’ve got a son instead of a daughter, you should know about a single factor that makes him eight hundred times more likely to play in the majors than a random boy.

What could possibly have such a mighty influence?

Having a father who also played Major League Baseball. So if your son doesn’t make the majors, you have no one to blame but yourself: you should have practiced harder when you were a kid.

That said, there is a rather large body of literature on the topic of birth month and its relationship to later outcomes. Which is why it's interesting to see a paper (working version here), just published in The Review of Economics and Statistics, which offers a different angle on all this birth-month conversation.

The Law of Unintended Consequences

There’s a natural ratio of men to women for our species, and it is not equal. For every 100 girls, 105 boys are born. But in some places, like India and China, the ratio is skewed. One Chinese city recorded an astounding 163 boys born per 100 girls. So, why is this happening?

The ultrasound.

The expanding use of this technology has allowed expecting parents to abort unwanted girls and keep the boys. The ability to sex-select has caused the disappearance of an estimated 160 million girls in Asia alone.

In this Marketplace segment, Stephen J. Dubner reports on the unintended consequences that come with new technology.