Indeed, the Freshman 15 is largely folklore, known perhaps more for its alliterative allure than its scientific veracity. ...
In America, first-year weight-gain was originally known as the "Freshman 10," and it was presumably adjusted upward as Americans got bigger.
"She appeared to have what is known here as the 'freshman 10,' the 10 pounds many freshmen gain in their first weeks," the New York Times sniffed in 1981 in an article about the first year at Yale for "Miss Foster"—Jodie, that is.
The first article to reference the Freshman 15, meanwhile, was Seventeen magazine in 1989... Before that, the only medical research to mention first-year weight-gain was a 1985 Addictive Behavior study in which the subjects gained an average of just 8.8 pounds. ...
A 2011 study found that having six or more drinks on at least four days per month was the only thing that made a significant difference when it came to keeping one's high-school figure. Even then, the drinkers gained just a pound more than non-drinkers did.
That same study found that in reality, just 10 percent of college freshmen gained 15 or more pounds, and a quarter of them actually lost weight. Instead, college students gain weight steadily throughout their time in school—women gain between seven and nine pounds total, and men gain 12 or 13.
Furthermore, the increase seems to be a natural part of adulthood, not something unique to dorms and dining halls. College freshmen gain just half a pound more than people their age who don't attend college.
And this is not an isolated finding: A 2008 study found an average weight gain of 2.7 pounds. A 2014 study found no change in college students' BMIs between the time they were admitted and the time they graduated. A total of 1858 subjects followed in 14 different studies averaged a gain of just 4.6 pounds during their first years. None came anywhere close to 15 pounds.
--Olga Khazan, The Atlantic, on an alliteration too good to check