Large-scale cheating has been uncovered over the last year at some of the nation’s most competitive schools, like Stuyvesant High School in Manhattan, the Air Force Academy and, most recently, Harvard.

Studies of student behavior and attitudes show that a majority of students violate standards of academic integrity to some degree, and that high achievers are just as likely to do it as others. Moreover, there is evidence that the problem has worsened over the last few decades.

Experts say the reasons are relatively simple: Cheating has become easier and more widely tolerated, and both schools and parents have failed to give students strong, repetitive messages about what is allowed and what is prohibited.

“I don’t think there’s any question that students have become more competitive, under more pressure, and, as a result, tend to excuse more from themselves and other students, and that’s abetted by the adults around them,” said Donald L. McCabe, a professor at the Rutgers University Business School, and a leading researcher on cheating.