By Stephanie Daly

Infidelity in relationships is an occurrence some college students can relate to, including 19-year-old Jon Cybulski, who said his ex-girlfriend cheated on him after a two-year relationship.

“I have never felt so mistreated in my life by someone that I cared about,” he said about the break-up.

Cybulski, a sophomore behavioral neuroscience major, said he met his girlfriend in high school and they continued to date when he came to Northeastern. After he found out she cheated on him with two men and lied about it, he said he ended the relationship.

According to a study done by graduate student Genevieve Beaulieu-Pelletier at the University of Montreal, cheating during the course of a relationship varies can range from 40 to 76 percent in each relationship.

Northeastern psychology professor Emily Kales said there are some social reasons people cheat, including the acceptance of experimentation in society, unhappiness in relationships and self-destructive behavior.

“I think biologically some people might argue that we are not designed to be monogamous,” Kales said.

Nancy McGrath, a sex therapist who has had a private practice in Brookline for 20 years said people make agreements not to have sex which might not be the best way to approach a relationship.

“If you’re generating inside of you your own monogamy, that’s better than making an agreement with another person.”

The University of Montreal study also found that among people who were an average age of 23, 41 percent admitted to cheating. Among people with an average age of 27, the percentage was 39.

Beaulieu-Pelletier said that given the study’s margin of error, these percentages are considered the same.

She said the study found a relationship between sex and attachment avoidance, a condition where people have difficulty making commitments.

“If avoidant people have difficulty with proximity and engagement in their relationship, when their partner gives them a sign of engagement they feel pressure from their partner, and one way to regulate the frustration is infidelity,” she said.

Some students said the perception of infidelity in society may come into play in its frequency in relationships.

“Society has really conflicted views. On one hand, we’re quick to say infidelity is wrong, but on the other hand we are quick to break relationships,” said middler sociology major Sarah Stokey.

Cybulski said age and the fact that he was in college, played a role in his break-up.

“We started dating so young. I sort of feel it’s hard for someone to grow with a person when yo”re dating. In college that’s the time when you try to find yourself,” he said.

Gina Ogden, a sex therapist from Cambridge and author of “The Return of Desire,” said she also thinks age is a factor.

“When you”re young, when you’re in your 20s, it makes sense that you’re going to be having different partners. You’re likely to be meeting a whole lot of people and dating,” she said.

While experts said there are reasons for cheating, it still affects people negatively, they said.

“The one who feels their partner is being unfaithful feels rejected and angry, and the one having the outside relationship probably feels guilty,” Ogden said.

Cybulski said the worst part of his break-up was that his ex-girlfriend admitted if he hadn’t found out about it, she probably would have never told him.

“It was the worst feeling ever because I put so much into the relationship. I sacrificed so many things in college and blindly thought everything was perfect,” he said.