New research may help explain why people find it so easy to fall back into old habits after they’ve decided to change their ways.

Longtime habits, such turning to food when you’re blue, may become so automatic that we need to constantly stay on our toes to keep them at bay, the study suggests.

In word-association tests given to 48 college students, researchers found that the word pairs the participants learned first seemed to hold a special place in their memories.

Under conditions where they had a choice of responses to give to a cue word, they tended to automatically go for the one they originally learned.

“It’s analogous to going to the refrigerator and training yourself to get the carrots instead of the cake,” said lead study author Dr. Cindy Lustig of the University of Michigan in Ann Arbor.

If you’re not vigilant about reinforcing your new commitment to those carrots, your hand is likely to go to the cake, according to Lustig. This may be especially true in times of stress or fatigue, she told Reuters Health.

Lustig, who was with Washington University in St. Louis at the time of the study, and her colleagues report the findings in the journal Psychological Science.

For the study, participants first learned to match a particular word to its cue word - saying “cup” when shown the word “coffee,” for example. Later, they were given a second group of response words to learn; in the coffee example, they were told to respond with the word “mug” instead of “cup.”

The students took one of two tests soon after learning the words, then again one day later. In one test, respondents were told to try to remember the original word they learned for each cue word. The other test looked at automatic memory, asking students to respond to each cue with whichever word came to mind.

On the second day of testing, students taking this latter test were much more likely to respond with the word they originally learned than with the one they had learned more recently - that is, “cup” was a more popular response than “mug.”

That contrasted with their first-day performance, when they responded with the original and newer words at roughly equal rates.

On the other hand, test-takers who were told to concentrate on remembering the “correct” response word performed similarly on both test days.

According to Lustig, the findings illustrate that it takes continuing hard work to keep old habits from cropping back up.

Weight watchers may want to ramp up their vigilance on Thanksgiving. Many of us stuff ourselves on that day, Lustig noted, “not just because it tastes good,” but also because of ingrained habits associated with the holiday.

SOURCE: Psychological Science, November 2004.