The "unconscious thought" theory for making complex decisions was proposed in a 2006 study by Ap Dijksterhuis at the University of Amsterdam in the Netherlands, and colleagues.
The team showed volunteers a series of cars and their attributes on a screen, before asking half of them to think carefully about choosing the best car, and the other half to solve anagrams - a distraction technique to allow unconscious processing.
Those in the anagram group were more likely to choose the cars with the best attributes, leading the researchers to conclude that it is best to leave tough choices to the unconscious (Science, vol 311, p 1005).
Now two teams have questioned this conclusion. Instead, they suggest that the volunteers made their decisions when they first viewed the data, based on an immediate gut instinct. Those in the anagram group simply recalled this original decision when asked to choose.
Those in the "thinking" group, however, reconsidered their first impressions while the details of the cars faded from their memory, which led to poorer choices.
"What Dijksterhuis ignored is that people might already decide when they first hear about the cars, and not after thinking about it or solving anagrams," says psychologist Daniel Lassiter of Ohio University in Athens.
To test this hypothesis, Lassiter and his colleagues repeated Dijksterhuis's experiment with a twist: they told the volunteers to memorise the cars' attributes while viewing them, thus distracting their attention from making an immediate decision.
The small tweak made a big difference. In contrast to Dijksterhuis's experiment, students made better choices when they spent time thinking, rather than solving anagrams (Psychological Science, vol 20, p 671).
Lassiter says this is strong evidence against the idea that unconscious deliberation is superior to conscious decision-making. He questions whether unconscious thought exists at all.