"Because asexuality is a relatively new phenomenon that has been described -- not that it hasn't existed for many, many centuries -- people don't understand what it is," Brotto said. "Because most people can identify with the feeling of sexual attraction, the notion that someone would not have sexual attraction toward anyone seems bizarre."
Hints of the existence of asexuality have appeared in the scientific literature since the 1940s. But it was not until more than a half century later that Anthony Bogaert, professor and chair of the department of community health sciences at Brock University in Ontario, Canada, took a closer look at those who professed to have no sexual attraction whatsoever to either men or women.
Bogaert's 2004 study is viewed by some as the first solid toehold for asexuality in the spectrum of sexual orientation -- a group which until recently had been comprised only of three categories: heterosexuality, homosexuality and bisexuality. In it, Bogaert looked at data from a survey of more than 18,000 British residents and examined their answers on a particular question on sexual attraction to others. While five of the possible answers to the question focused on varying levels of attraction to males or females, the sixth answer that respondents could choose read "I have never felt sexually attracted to anyone at all."
"About 1 percent of individuals reported having no sexual attraction to anyone at all," he said. "This was the missing fourth category of sexual orientation."
What followed this finding was much discussion over whether asexuality should be seen as a distinct sexual orientation or treated as a pathological condition -- a debate that largely persists until today.
Prior to this research, and even until today, asexual tendencies were generally assumed to be a sign of hypoactive sexual desire disorder (HSDD) -- in other words, a low sex drive. It is a distinction with which the psychological community still wrestles.
"It is very hard to distinguish between asexuality and insufficient capability for desire or arousal or both," said Pepper Schwartz, professor of sociology at the University of Washington in Seattle. "Perhaps the difference is how the person -- or their partner -- feels about it. The literature is certainly inadequate for differentiation between the two."
Bogaert said that a lack of widespread understanding about asexuality means that many of these individuals face many challenges -- particularly in a society that seems fixated on sex.
"A big part of our media and culture expects people to have romantic/sexual relationships with others," he said. "The norm is for someone to form romantic sexual relationships with other people."
This norm, Bogaert noted, often poses problems for asexuals, who may be interested in romance and intimacy, but not in sex.
"[Asexuals] may want to pair-bond with another individual, and most likely they will be pair-bonding with someone who's sexual," he said. "So then you often have pressure placed on the asexual person to have sex within the relationship, even if he or she really doesn't want to have sex."
Eli Coleman, professor and director of the program in human sexuality at the University of Minnesota Medical School, said that this clash of expectations could lead to serious relationship stress.