Madeleine Watson, 29, a resident of Vancouver, British Columbia, said: “I’ve been to, maybe, three or four house parties in the last couple of years, and they’re always Halloween or New Year’s or a holiday event. And it’s always the same house and the same people throwing it.”

Christine Vines, 27, said in her first couple of years in Brooklyn, she and her roommate hosted, maybe, two house parties. “That was a trial,” she said. “We decided it was more effort than it was worth. I went to a handful a year, usually Halloween or New Year’s.”

Sneeze at these comments as anecdotal evidence cherry-picked for another nebulous hypothesis about millennials, or cite the all-night rager you attended last weekend as disproof. Yet some data does suggest that 20-somethings are scaling back.

According to the Bureau of Labor Statistics, the average number of hours per day 15- to 24-year-olds spent attending or hosting social events on weekends or holidays — the times they are most likely to go to parties — declined sharply from 2003 to 2014 to nine minutes from 15. (That may not seem like much, but consider that this is the average of all those who fit the demographic.) The percentage who participated in these activities dropped to 4.1 from 7.1 over the same span.

Their tame night lives began in high school. According to a nationwide annual survey by the University of California, Los Angeles, the time high school seniors devoted to partying has slid drastically over the decades. Except for a few years, the number of homebodies who never attended parties as high school seniors has steadily increased, to 41.3 percent in 2014 from 11.6 percent in 1987, and it’s accelerated in the new millennium, more than doubling since 2001. Over a third of Gen X high schoolers fought for their right to party at the tail end of the Reagan administration, spending more than six hours per week at gatherings; just 10.7 percent of the most recent Obama-era high school seniors did.