A couple of days ago, David Brooks had a column in which he wrote "In the 1980s, 20 percent of Americans said they were often lonely. Now it’s 40 percent." I've seen research on changes in the number and type of ties among people, but I didn't know of anything on feelings of loneliness, so I tried to investigate further. Brooks didn't provide a link to his source, but a Google search showed other articles making the same claim. The source of the 40% figure seems to be a survey of people aged 45 and over sponsored by the AARP in 2010. However, the report of that survey didn't say anything about changes in loneliness.
There is a question that has been asked in a number of surveys asking people if they had felt "very lonely or remote from other people" in the past few weeks. The percent saying they had:
Nov 1963 28%
June 1965 26%
Jan 1981 17%
May 1990 19%
Sept 2001 26%
Dec 2001 24%
That doesn't look like any kind of trend. The numbers in the 1981 and 1990 are lower, but they were in surveys taken by Gallup, and the others were by NORC, so that may be a factor. Unfortunately, the question hasn't been asked since 2001.
I searched Google scholar for papers about trends in loneliness, and found one from 2014 entitled "Declining Loneliness Over Time: Evidence From American Colleges and High Schools" . It was based on surveys at various colleges and universities and on the Monitoring the Future Survey, a representative survey of high school students that has been conducted since the 1970. It mentioned that other literature claimed that loneliness had increased, but I checked the sources they cited and they didn't provide any evidence--they just said it had, or cited research that wasn't really relevant.
It's possible that I missed something, but I doubt that there is any actual evidence that feelings of loneliness have doubled since the 1980s. My guess is that the claim is based on a widely cited paper published in 2006, "Social Isolation in America: Changes in Core Discussion Networks over Two Decades," which found that the percentage of people saying that in the last six months they had not "discussed matters important to you" with anyone went from 10% in 1985 to 25% in 2004. They called this "social isolation," which sounds more or less equivalent to "loneliness," so you can see how one would turn into the other. The change in discussion networks for "important matters" is interesting, if it happened (as the authors acknowledge, it might be at least partly an artifact of survey procedures), but it's not necessarily the same as a change in feelings of loneliness.
1. It's remarkable that online editions of newspapers and magazines haven't developed reasonable conventions about when to include links to a source. I checked five or six articles, all in well-regarded publications, which included the claim that levels of loneliness had doubled. Only one provided a link: that was to the AARP survey report, which didn't support the claim.
2. There are cases when you can't say much about trends because there are recent survey questions, but no older ones. This isn't one of them: in addition to the "very lonely or remote," there was a 1964 survey asking people to agree or disagree with the statement "I often feel quite lonely" (27% did), and a 1990 Gallup Poll asking "How often do you ever feel lonely?" (10% frequently, 26% sometimes, 40% seldom, and 23% never) and a number of related questions. There is also a Gallup question from 1950: "When you have personal problems, do you like to discuss them with anyone to help clear them up, or not?" and a follow-up about who you discuss them with.
[Data from the Roper Center for Public Opinion Research]