LOS ANGELES — Good-looking celebrities are no more effective as product spokespeople than unattractive celebrities - unless the product is one that consumers hope will enhance their own attractiveness. In a University of Southern California study, students rated 10 products for enhancing one's attractiveness. Luxury cars ranked highest and computers lowest. Then the students selected two celebrities, one considered attractive, the other not. The students found that the attractive celebrity was more believable and likable than an unattractive one in a luxury car ad. But when endorsing computers, the two celebrities were rated the same.