An unfortunate feature of science is that two experiments that are ostensibly looking at the same thing can produce different results. Often, the different results are greeted, unhelpfully, with the experimenters—and sometimes even the entire field—being accused of being garbage. A more helpful response is to consider why the experiments, while looking at the same thing, might not be identical and whether the differences between them might tell us something.

A new study in Nature Human Behavior describes a subtle way some psychology experiments could differ: if they include breaks to let their participants avoid tiring out. Enforced breaks can cause people's moods to drop and continue dropping if the break drags on. And, since mood has been shown to affect behavior in a variety of other psychological tests, this has the potential to have a complicating influence on a huge range of studies.

Waiting is the hardest part

The work began with an incredibly simple finding. Most studies operate under the assumption that a participant's mood remains relatively stable throughout an experiment. But the researchers here asked participants to rate their mood at the start and end of experiments—and thus at the start and end of breaks between the experiments. The researchers noticed that the mood went down pretty consistently over the course of the break. After a roughly 10-minute break, people assessed their mood as more than 20 percent lower than when the break started.

At this point, the researchers decided that the effect, which they term "mood drift over time" was worth looking into. So they took various steps to ensure the quality of their work, pre-registering their study designs and getting many participants involved in replication experiments. All told, nearly 30,000 participants, both adults and adolescents, took part in these experiments.

Some of these tests ruled out the possibility that being repeatedly prodded about their mood worsened people's mood. And they tried several different mood assessment methods and found that the details didn't make a difference: mood got worse no matter how they asked. They also got a mood sample from many people who played a risk-taking game on a mobile app. People were doing that for fun, so the effect was smaller, but being told to wait between games still made people feel down.

The one instance they found that didn't make people feel worse was replacing the enforced break with telling people they could go off and do whatever they wanted for seven minutes. Even though the self-reported activities were pretty minor—things like standing up, thinking, and skimming the news featured prominently—this was enough to avoid a statistically significant change in participants' mood.

Drifting

The last result suggests that it's not so much inactivity as a loss of a sense of agency that bothered people. Loss of agency is also a factor in boredom, which could seem to account for the drop in mood. But an experiment that also asked people to rate their boredom showed that, while there's some overlap between boredom and mood drift, the former doesn't fully account for the latter.

By looking at participants who were involved in more than one of these studies, the researchers also found that the magnitude of the mood drift was fairly consistent in an individual over time, suggesting that the effect may be at least partly a stable personality trait.

Finally, the researchers showed that this spills over into behavioral tests. There are lots of tests that look at risk tolerance; the tests can implement a high probability/low payoff combination or a high risk/high payoff gamble. Using one of these tests, the researchers found that people were less likely to gamble after they experienced mood drift.

Overall, the researchers present their work as a caution. It's likely that there are a wide range of behaviors influenced by our overall mood, and many of those have likely been the subject of experiments. Mood drift means that the precise structure of the experimental routine can influence the outcome of those experiments, even if the exact same tests are being used. In other words, it could explain why two seemingly similar studies could produce different results.

It also may go a long way toward explaining why people hate to be made to sit still in a room.

Nature Human Behavior, 2023. DOI: 10.1038/s41562-023-01519-7  (About DOIs).