Was It Last Month or the Month Before? What a 1980 Volcano Showed About Dating Memories

Ask someone whether they were robbed in the last six months and some of the yeses will describe something that happened eight months ago. Memory researchers call this forward telescoping: an event is remembered as nearer to the present than it was. It is a dating error, not a forgetting error, and the best-known fix is to stop asking about “the last six months” and ask about “since” something everyone can date. In 1983 Elizabeth Loftus and Wesley Marburger of the University of Washington tried it with a volcano.

The experiment with the volcano in it

Mt. St. Helens erupted on May 18, 1980. Six months later, on November 17, 18 and 19, Loftus and Marburger approached 144 people in and around Seattle (ordinary citizens and passengers waiting at the airport) and asked about crime. Seventy-six got questions in the usual form: “In the last 6 months, did anyone try to rob you?” Sixty-eight got “Since the first major eruption of Mt. St. Helens, has anyone tried to rob you?” The two windows were the same length. Only the way the beginning was marked differed.

Seven of the 76 people in the “6-months” group (9.2%) reported at least one victimization. One of the 68 in the eruption group (1.5%) did (z = 2.03, p < .05). The authors then probed the seven. Two placed their event in the summer, safely inside the window. Two said it was before the window. Three, once asked, were unsure whether it came before or after May 18. All but one person in the eruption group could date the eruption as May 18, mid-May or “in May.”

That is a small sample, and the paper says so (“admittedly small”). The idea went into a series of five experiments with 1,694 subjects in total. In Experiment 4, interviewers at five Seattle shopping areas asked 498 people either about “the last 6 months” or “since New Year’s Day” at the end of June 1981: 16 victimizations were reported with the first wording and 8 with the second. Experiment 5 phoned 720 people in June and July 1982. With “last 6 months,” 20 people (8.3%) reported something; with “since New Year’s Day,” 8 (3.3%).

Experiment 5 had a third group, which is the part I find the most interesting. They were asked about “the last 6 months, that is since” a specific date exactly six months earlier. That group came out in between: 15 reports (6.3%). So some of the landmark’s benefit is just that it forces a precise date into the question, and some is left over, which the authors attribute to the landmark itself. The paper stops short of explaining why. One guess they raise is that a landmark turns a hard absolute judgment (“how long ago?”) into an easier relative one (“before or after the eruption?”), but they present it as a possibility, not a finding.

Dating is guesswork that gets worse with distance

Loftus and Marburger described the error. A different group of researchers explained where it comes from. Rubin and Baddeley (1989, Memory & Cognition) proposed a model in which nobody’s dating is biased in a particular direction, and the telescoping falls out of three ordinary facts: recent events are remembered better than old ones, dating errors grow the longer ago the event was, and the events that sneak into a reported window come from before it, not from the future. In their first experiment, psychologists’ errors in dating colloquia over a two-year span grew at about 0.4 days per day of delay, and the errors pointed toward the middle of the interval. The title makes the claim in six words: “Telescoping is not time compression.”

Janssen, Chessa and Murre (2006, Memory & Cognition) looked at how people date events from the news and from their own lives. They found a small backward telescoping effect for recent news events and a large forward one for remote events. Dates given in an absolute format (“July 2004”) were more accurate than dates given as “3 weeks ago,” yet participants preferred the relative format for news and for remote events. In other words, the way most of us naturally say when something happened is the less accurate way.

What this does and doesn’t say about blurry weeks

It is tempting to fold this into the familiar claim that routine makes weeks vanish. They are different effects. The feeling that a week was long or short, and why it shrinks in memory when every day is alike, is a question about duration; I went through that literature in a piece on why a week away feels longer than a month at home. Telescoping is about where a particular event sits on the timeline. The two probably feed each other, because sameness leaves few distinctive moments to hang a date on, but none of the three papers above tests that link. Loftus and Marburger asked about crimes, Rubin and Baddeley about talks, Janssen and colleagues about news and personal events. Nobody in them studied identical workdays.

Two cautions from the papers themselves. First, the landmark results are about survey respondents answering for a window of about six months, with Seattle residents who knew the eruption date. A landmark nobody can date would not help. Second, in the experiments I read, fewer reports were treated as fewer telescoped reports; the strongest check was the follow-up probing in Experiment 1, which was a handful of people.

Using the idea on your own calendar

Some things follow if you accept that dating gets less reliable the further back you go. If you want to know when something happened, a record made at the time beats your sense of it. Asking “did that come before or after the trip?” is easier than asking “how many weeks ago?” And if you are keeping a diary or running a self-experiment, write the date down rather than the relative guess. The same reasoning is behind the advice in preregistering a 30-day self-experiment to record each result at the moment it happens: a rating written on the day is a dated fact, while the same rating recalled a month later is a guess with a date on it.

Blurry weeks are also what the doorway effect and event boundaries make more likely: a day with no boundaries in it gives memory no landmark to file things against. If you want a cheap landmark, one that is recorded and dated, a morning clip counts. In DontSnooze (iOS), the video you record when the alarm fires goes to your group that morning, as described in how DontSnooze works, which leaves your friends holding a dated trace of one thing about that day. Nothing in the research above says this improves anyone’s memory; it is simply a record that does not depend on remembering.

Questions people ask about telescoping

Is telescoping the same thing as time speeding up as you get older?

No. Telescoping is an error in dating a single event, usually placing it nearer to the present than it was. The age question is about how fast a stretch of time feels. Rubin and Baddeley (1989) showed that dating errors can arise without any speeding up or compression of time at all; the effect can come from memory fading for older events and from errors that grow the longer ago something was.

Should I use “since the clocks changed” instead of “in the last month” when I try to remember?

It is a cheap thing to try, but the evidence is about survey respondents, not about private recall. In Loftus and Marburger’s five experiments a landmark such as an eruption or New Year’s Day cut the number of events reported inside the window. The authors also found that part of the gain came simply from the landmark being a precise date, so a firm calendar date may do some of the same work.

Do these results mean fewer reports are always more accurate?

Not automatically. The authors treated a drop in reports as evidence of less forward telescoping, supported by follow-up probing in Experiment 1, but a landmark could in principle also make people stricter about what counts. The samples were convenience ones (airport passengers, shoppers, students, phone respondents), and Seattle residents knew the eruption date; a landmark nobody can date would not work the same way.

Sources

Loftus, E. F., & Marburger, W. (1983). Since the eruption of Mt. St. Helens, has anyone beaten you up? Improving the accuracy of retrospective reports with landmark events. Memory & Cognition, 11(2), 114-120 (full text read). Rubin, D. C., & Baddeley, A. D. (1989). Telescoping is not time compression: a model of the dating of autobiographical events. Memory & Cognition, 17, 653-661 (abstract read). Janssen, S. M. J., Chessa, A. G., & Murre, J. M. J. (2006). Memory for time: how people date events. Memory & Cognition, 34, 138-147 (abstract read).