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America is having an addiction moment. Media headlines scream daily about new neuroscience findings on porn addiction, Internet addiction, food addiction and plain, old-fashioned drug addiction. As TIME's Michael Scherer wrote last week, 42 U.S. Senators called on Attorney General Eric Holder to increase obscenity prosecutions for pornography, due in part to fears over sex addiction. There's even a new online magazine about addiction called The Fix (full disclosure: I write for it).

But what do the recent neuroscience studies actually tell us about addiction? What is addiction really? An article on the website referenced by the U.S. Senators in their letter to Holder explained sex addiction this way:

Cocaine, opioids, alcohol, and other drugs subvert, or hijack, the brain's pleasure systems, and cause the brain to think a drug high is necessary to survive. Evidence is now strong that natural rewards such as food and sex affect the reward systems in the same way drugs affect them, thus the current interest in 'natural addiction.' Addiction, whether to cocaine, food, or sex occurs when these activities cease to contribute to a state of homeostasis, and instead cause adverse consequences.

But this is actually a bizarre form of circular reasoning. For one thing, it confuses the original purpose of the brain's pleasure pathways. If you think about it, these regions aren't very likely to have evolved specifically to enable us to get high on cocaine or heroin — that would be evolutionary sabotage. But they almost certainly did evolve to get us to pursue food and sex relentlessly, in order to guarantee our survival. So, these brain regions are designed to make food and sex fun. (More on TIME.com: Addiction Files: Recovering From Drug Addiction, Without Abstinence)

Suggesting that drugs "hijack" these reward systems to produce addiction implies that under normal circumstances — in the pursuit of "healthy" urges like eating and sex — these systems wouldn't cause addiction. Except of course the same reasoning, turned around, is used to argue that in fact food and sex are addictions — because they activate the brain regions that generate pleasure from drugs.

All that can actually be said based on this data is that food, sex, drugs — and anything else that's fun — activate brain regions and cause pleasure, and that people enjoy and pursue pleasure. However, that does little to explain why said brain activity can sometimes cause addiction, but mostly doesn't. The vast majority of people aren't food or drug addicts — and it's not clear what's different about the minority who are. (More on TIME.com: Does Men's 'Bond' with Porn Ruin Them for Real-Life Sex?)

So pleasure alone isn't addiction — and not even craving plus pleasure are enough to define addiction. The most accepted definition, as implied by the link above, describes addiction as compulsive behavior related to a substance or activity that people continue to engage in despite negative consequences. That's the essence of the definition used by the DSM, psychiatry's diagnostic manual.

It's a complex phenomenon that goes beyond mere activation of pleasure pathways. Unfortunately, most animal and brain-imaging studies tell us very little about it. Last week, for example, I reported on a study about "food addiction": researchers found that when women were presented with a chocolate milkshake, those who had more symptoms of food addiction had greater activity in brain regions linked with pleasure and craving, and reduced activity in areas connected with self-control, compared with those who had few or no food-addiction symptoms.

That's interesting, but again all it really shows is that symptomatic participants predictably reported more pleasure and craving, and less self-control. And it's important to note that none of the women in the study actually had food addiction; some had symptoms, but none had full-fledged eating disorders. The brain differences shown in the study, therefore, were seen in perfectly normal people — most of whom, prior research suggests, will never be addicted to food. (More on TIME.com: Heroin Versus Häagen-Dazs: Food Addiction and the Brain)

No study has ever isolated a simple brain change that is always seen in addicts and never in non-addicts. And although some studies have found changes that can help predict an addict's odds of relapse, they're not always accurate.

This failure to define or understand the blurry line between the normal and the pathological leads to all kinds of misconceptions about addiction: for example, thinking that simple exposure to a substance or experience is enough to create addiction. Most of us enjoy the occasional chocolate milkshake, but virtually none of us would rob a bank to get one if the price became unaffordable. Similarly, more than 97% of people who take opioid painkillers like Oxycontin as prescribed, and don't have a prior history of addiction, do not become addicted, studies show. (More on TIME.com: Why French Fries Are Such Good Comfort Food)

And as Virginia Heffernan pointed out in the New York Times this week, college students who define themselves as "Internet addicts" don't actually behave very differently than their predecessors who were also occasionally known to blow off homework and stay up late in pursuit of a hobby or interest.

Addiction research is continually filling in parts of the puzzle of addictive behavior. But to use existing evidence to claim that sex and food create overpowering drives by "hijacking" the brain's pleasure pathways doesn't tell the whole story. We need to understand a lot more about how we control and choose our behavior ordinarily before we can truly know what causes our decisions to go awry in addiction. If we don't understand how food and sex normally affect our brains and our choices, how can we understand what triggers addictions to any substance or experience?

• Maia Szalavitz