From Sleeping on it best for complex decisions:
In one of the tests, half of the participants were asked to ponder on the information they were given and then decide which among similar products to buy. The other half were shown the information but then made to perform a series of puzzles including anagrams and simple arithmetic. At the end of the puzzle session, the participants were asked to make a snap decision about the products.“We found that when the choice was for something simple, such as purchasing oven gloves or shampoo, people made better decisions — ones that they remained happy with — if they consciously deliberated over the information,” says Dijksterhuis.
“But once the decision was more complex such as for a house, too much thinking about it led people to make the wrong choice. Whereas, if their conscious mind was fully occupied on solving puzzles, their unconscious could freely consider all the information and they reached better decisions.”
However, the unconscious mind appears to need some instruction. “It was only when people were told before the puzzles that they would need to reach a decision that they were able to come up with the right one,” Dijksterhuis told New Scientist.
If they were told that none of what they had been shown was important before being given the puzzles, they failed to make satisfactory choices.
“At some point in our evolution, we started to make decisions consciously, and we’re not very good at it. We should learn to let our unconscious handle the complicated things,” Dijksterhuis says.