As more women continue to come forward with allegations that comedian Bill Cosby drugged and sexually assaulted or raped them, the question of why people commit such sex offenses has come under the spotlight. Cosby has never been charged with a sex-related crime, but the majority of the women who have come forward alleged that he drugged them before groping, assaulting or raping them. If the allegations were proven to be true, they would paint the picture of a serial sex offender with a predilection for using drugs and alcohol to lower his victims' defenses.

But sex is not the driving factor for most rapists, who instead seek to exert their power on others, sexual assault experts said. "Generally this has to do with issues of both power and control," Dr. Joseph J. Plaud, a clinical and forensic psychologist and executive director of Applied Behavioral Consultants in Whitinsville, Massachusetts, said in an email interview. "Most rapists are not sexual deviates. As the feminists have rightly pointed out for decades, most rapes are not sexual, but rather violent criminal activity, where sex (like guns, knives and other forms of physical coercion) is used as a weapon. Many rapists seek to dominate women, to control them, through acts of sexual aggression, including rape."

A 2007 study by the Medical University of South Carolina's National Crime Victims Research & Treatment Center found that during the previous year more than 1 million women in the U.S. were raped, more than 800,000 had been forcibly raped and nearly 200,000 experienced drug-facilitated rape. In recent years, complaints of college men using drugs to sexually assault fellow students has become a growing problem on campuses across the nation.

"Typically what we see is people who are using drugs to commit a sexual offense are doing so because obviously that increases the vulnerability of the victim and decreases the resistance," Dr. Nancy Zarse, a professor in the department of forensic psychology at the Chicago School of Professional Psychology, said. "The drugs also distort people's memories ... so they're afraid to come forward: 'Did that just happen, did he do that, would he do that?' Especially if a person is powerful, they may be worried that they won't be believed."

The National Crime Victims Research & Treatment Center study reached a similar conclusion: "One of the more striking findings of this study was that only 16 percent of all rapes were reported to law enforcement. Notably, victims of drug-facilitated or incapacitated rape were somewhat less likely to report to the authorities than victims of forcible rape."

Cosby, however, is accused of being a serial offender. Such criminals are most often afflicted by either antisocial personality disorder and/or psychopathy, Zarse said. While they have some important similarities, being diagnosed as antisocial and being labeled a psychopath have key differences, but they are both potential factors in any serial rapists' set of motivations, according to Zarse.

An antisocial person generally exhibits a range of traits including pervasive disregard for (and violation of) the rights of others, lack of empathy and remorse and repeat impulsivity, Zarse said. A psychopath generally exhibits some or all of the antisocial traits, coupled with the added traits of cunning, being manipulative, "a glib, superficial charm and a callous lack of empathy," Zarse explained.

"People who have antisocial personality disorder have a lack of remorse, they don’t empathize with the victim, and they either don’t think it damages the victims or they don’t care about the damage," Zarse said. "There are a lot of sex offenders who rationalize their behavior. It's called a thinking error, and they rationalize their behavior as not being bad. You often see that with pedophiles, they say they’re teaching a child about sex and think that’s a good thing."

As such, because of this lack of remorse, they often see no incentive to stop committing their crimes if they are not punished, and over a long period of time they can continue to repeatedly rape or assault, becoming a serial offender in the process.

"If individuals have such power and control issues with women, coupled with access and other forms of power (like money and celebrity), then this can become a potent elixir for sexual violence, because it becomes unchecked," Plaud said via email. "Individuals can go decades without being found out or in any way sanctioned for such aggressive/violent behavior with women. And behavior that does not receive a consequence from the criminal justice system may continue unabated."

Unlike a schizophrenic, who often has no control over his or her aberrant behavior, an antisocial person or psychopath generally has control over their actions, though they are seen as deep-seeded and pathological behavior. And often a sex offender or serial rapist has suffered some form of sexual trauma in their past, according to Zarse.

"We do see a high percentage of people who sexually offend who were sexually offended as children, but again we also see a lot of people who were sexually offended as children who don’t become sex offenders," she explained.