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Might the blossoming of fast food be making people more impatient generally? That is one possible conclusion from a study of two distant cousins of humans.

The research, led by Harvard psychologist Jeffrey Stevens, focused on the "discounting of future rewards" -- a principle that says animals generally place more value on a reward they can have right now than on the same reward promised for some future time.

Stevens' team studied the behavior of common marmosets and cotton-top tamarins in a setting that gave them a choice of getting two food pellets immediately, or six after varying delays.

On average, tamarins were equally likely to go for the two immediate pellets or wait for six when the delay was eight seconds. By contrast, the marmosets' tipping point was 14.5 seconds. Beyond those cutoffs, both tended to go for instant gratification.

Why the difference? The two closely related species are both New World monkeys with comparable body and brain sizes and similar social behaviors. But marmosets spend about 70 percent of their feeding time getting sweet, gooey gum from trees, while tamarins feed mostly on insects.

"Gummivory requires scratching tree bark and then waiting for the sap to flow, while insectivory favors immediate acquisition of an ephemeral food source," the team notes in Biology Letters, a journal of the Royal Society. They add that "foraging ecology" -- the way a species goes about getting food -- "may provide a selective pressure for the evolution of self-control."

-- Washington Post