The holiday season can bring out the worst in Christmas shoppers when it comes to irrational buying, according to a Miami University marketing expert.

"When we feel depressed by the season, it's not uncommon for us to go out and buy items we don't want or need, just to pick up our spirits," says Jack Gifford, chairman of the university's marketing department.Another holiday buying trend, he says, indicates shoppers also find it much easier to spend lots of money on several smaller items and avoid the one major ticket item.

For many busy families, according to another Miami University professor, stress is intensified during the yuletide season.

"Christmas is particularly stressful for two-income families attempting to balance the demands of jobs with personal expectations of family togetherness and holiday activities," says Timothy Brubaker, director of the university's Family and Child Studies Center.

"Addressing Christmas cards can be a major chore, much less baking and decorating cookies or entertaining, and the interruption in routines caused by school holidays adds to the tension," he says.

Brubaker, who edits "Family Relations," a journal of applied family and child studies published by the National Council on Family Relations, also believes holiday media images don't help with their portrayal of "traditional" families and career couples who are always at home.

Reality is much different, he says, noting that according to the 1987 census both mothers and fathers are employed in 57 percent of two-parent families with children 6 or under.

And then there is the matter of New Year's resolutions.

Many people find their will power to follow through on the resolutions begins to falter by early February because of a complex mind game called "self-handicapping," says Richard Sherman, professor of social psychology at the university.

"Self-handicapping gives people an excuse so that if a situation fails, it's not their fault," Sherman explains.