One of the most significant and persistent barriers to combating human trafficking is widespread insistence on distinguishing between sexual trafficking and prostitution. While linking the two is about as easy as connecting smoking to lung cancer, we continue to decouple trafficking from branches of the commercial sex industry like prostitution and pornography.

Of course most of us want to end sexual slavery, but the commercial sex industry — which is the very lifeblood of trafficking — is increasingly tolerated. Prostitution is seen by plenty as a legitimate, if suboptimal, form of "work," and pornography is taken to be harmless. And yet the commercial sex market and sex trafficking are symbiotically related; the latter simply would not exist without the former. Women and girls are trafficked into brothels, strip clubs and massage parlors. They're photographed and filmed servicing men. Customers — including men who simply click on free porn on the web — do not and cannot distinguish between trafficked women, prostitutes and porn stars.

Those who draw a bright line between sex trafficking and prostitution often argue for legalizing prostitution. After all, they say, if prostitution were legalized, then the government would be poised to regulate it and curb harms experienced by prostitutes. In fact, the State Department's 2010 Trafficking in Persons Report even claimed that countries that had legalized prostitution were making efforts to eliminate sexual trafficking.

But rather than eliminate sexual trafficking, the evidence has consistently revealed that legalizing prostitution fosters it. Dorchen Leidholdt, Co-Executive Director of the international NGO Coalition Against Trafficking in Women, stated:

"Jurisdictions that have legalized prostitution have demonstrated just what happens when prostitution is legitimized and protected by law: the number of sex businesses grows, as does the demand for prostitution. Legalized prostitution brings sex tourists and heightens the demand among local men. Local women constitute an inadequate supply so foreign girls and women are trafficked in to meet the demand. The trafficked women are cheaper, younger, more exciting to customers, and easier to control. More trafficked women means more local demand and more sex tourism."

In other words, sexual demand is not as stable as you might think; it can be stimulated. Just consider what happened in Australia when its government decriminalized prostitution and took control of the industry: "in New South Wales where brothels were decriminalized in 1995, the number of brothels in Sydney had tripled to 400-500 by 1999, with the vast majority having no license." In other words the illegal sector of the sex industry flourished once prostitution was legalized. The Netherlands are another excellent case study. Their brothels were legalized in 2000, but the number of reported human trafficking cases increased from 341 in 2000 to 909 in 2009. When the sex industry enjoys government protection, it thrives and demand increases. It also becomes much more difficult to identify instances of abuse and to prosecute trafficking.

What if a woman wants to become a prostitute? In her book "Prostitution, Power and Freedom," Nottingham University Sociology Professor Julia O'Connell explained that this phenomenon, known as "casual prostitution," accounts for a mere one percent of women in the sex industry. And in a recent study of trafficking and prostitution across nine countries, researchers found that out of 785 sex workers, "89 percent … wanted to escape prostitution but did not have other options for survival."

Free choice here is largely a myth. Catherine MacKinnon, pioneer of the legal battle against sexual harassment in the workforce, argued that "If being a sex worker were truly a free choice, why is it that women with the fewest options are the ones most apt to 'choose' it?" Closely related to the issue of choice is that of consent, or the idea that prostitution is innocuous if the prostituted woman gives her consent. But the condition of consent is an unfounded criterion.

Melissa Farley, director of the organization Prostitution Research and Education, explains that "it is a clinical, as well as a statistical error, to assume that most women in prostitution consent to it. In prostitution, the conditions which make genuine consent possible are absent: physical safety, equal power with customers, and real alternatives." While no doubt some women choose this line of work freely, they remain a very small minority.

When you operate within the framework of consent and choice-based rhetoric, there will still be women who meet the requirements for victim status but remain overlooked. This is one reason why the prevalence of sexual trafficking is underreported in the U.S. The Trafficking Victims Protection Act (TVPA) currently stipulates that in order to prosecute traffickers and receive aid themselves, victims must be either under 18 years of age, or prove that their entry into the commercial sex industry was the result of force, fraud or coercion. But what happens to women who initially agreed to come to the United States to work in the commercial sex industry (migrant sex "workers"), but would never have given their consent had they known what slave-like and abusive conditions awaited them? These women technically qualify for benefits under the TVPA but will have a very difficult time securing them since they can't easily prove that coercion occurred as defined by the law.

If the United States wishes to combat modern slavery, it should make exploitation, rather than force, fraud or coercion the main consideration in possible trafficking cases. The United Nations already does this and considers "consent" to be irrelevant in determining trafficking victim status. If the United States were to shift its emphasis away from proving force, fraud or coercion toward establishing whether persons were being exploited for commercial sexual services, then far fewer victims would fall through our legislative cracks, and it would be easier for law enforcement to prosecute sexual trafficking.

The fight for abolition is far from over. But it has at least begun. For instance, in September Craigslist shuttered its adult services section after charges that it was being used to facilitate sexual trafficking. (Unfortunately, we have no doubt that others will fill the gap.) Also, in 2010 alone more than 40 human trafficking bills were enacted with over 350 introduced, in comparison to a mere eight human trafficking bills passed into law in 2006. And over 850 people were arrested in a three-day November 2010 sweep aimed at suppressing child prostitution. Clearly our government is taking steps to combat human trafficking, but it remains hampered by underreporting, underdefining and a continued disinterest in linking sex trafficking to the larger commercial sex industry.

Ellyn Arevalo is a research assistant with the University of Texas at Austin's Population Research Center and lab manager for the university's human development and family sciences department. Mark Regnerus is an associate professor of sociology at the University of Texas at Austin. He is the author of "Premarital Sex in America: How Young Americans Meet, Mate, and Think about Marrying" (Oxford, 2011). The original and lengthier version of this article appeared in Public Discourse an online journal of the Witherspoon Institute.