Why large age gaps in marriages are declining

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(CNN) — A while ago, my friend told me a 30-year-old friend of ours was engaged to marry a 50-year-old.

The large gap made my mind think about the old -- and some might argue sexist -- "half plus seven rule," which has shaped cultural understandings of what is an acceptable age gap in relationships.

It's a simple math equation that most often comes up these days in references to relationships involving older men and younger women. You take the older partner's age (50 in the example above), divide it by two (25) and add seven (32). If that number is smaller than or equal to the younger partner's age, "the rule" suggests that the younger partner is old enough for the older partner to date. And if it's higher, that suggests the younger partner is too young.

Interpreting this "rule" as an ironclad law of relationship physics is somewhat ridiculous -- and outdated. Still, as I explore in my new "Margins of Error" podcast, age gaps in relationships have actually shrunk, despite all the press about celebrity couples with large age differences. In fact, according to data from the U.S. Census Bureau, the percentage of new female-male marriages that are breaking the rule has declined from 30% at the beginning of the 20th century to a little more than 10% in 1980 to a mere 3% today.

The falling share who break "the rule" reflects a society in which women and men are both becoming better educated and becoming more economically secure. This has led to people (especially women) getting married later.

So how did this so-called rule even come about? A deep dive into the archives shows that it's only in the last 25 years that "the rule" came to reflect the minimally acceptable age in a relationship. It used to be that half the man's age plus seven was considered an ideal gap.

This standard was first bandied over 100 years ago by men, who did not base their findings in statistics, but rather the gender power dynamics of the day. According to economics professor Terra McKinnish, "the rule" made sense under the male breadwinner model where "the man wants to get established in the labor market to be able to show the earnings potential. And so it makes sense ... to delay marriage a little bit so they can send that signal."

In essence, social norms called for the man to provide the money and the woman to provide the children.

As for contemporary relationships, the data for same-sex male couples tells a different story than it does for heterosexual couples. Men in same-sex marriages are far more likely to break the ​current incarnation of "the rule" than heterosexual couples: 15% compared to that 3% figure I cited earlier.

We'll explore the reason for that difference with relationship expert Dan Savage on the podcast.

Get the latest episodes of "Margins of Error" as soon as they drop. Find it in Apple Podcasts, Spotify or your favorite podcast app.

This story was first published on CNN.com Why large age gaps in marriages are declining