Enlarge (credit: Klaus Vedfelt)

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 as the experimenters—and sometimes even the entire field—are accused of being garbage. A more helpful response is to consider whether the experiments, while looking at the same thing, might not be identical. And, if they're not, 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 affects 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.