As you’d probably guess, rats don’t like the smell of death.

But a researcher named Jim Faust wondered whether that instinct could be changed, so he sprayed female rats with a liquid that smelled like a dead, rotting rat. When he put them in cages with virgin male rats, a strange thing happened. The drive to mate was so powerful that it overcame the instinct to avoid the smell, and the rats hit it off. Actually, that’s not so strange. The strange part was what happened next.

Once the male rats had learned to associate sex with the smell of death, Faust put them in cages with different objects to play with. The male rats actually preferred to play with the object that smelled like death, as if it were soaked in something they loved! [1]

We know what you’re thinking: “Now I know what I should have done for my science fair project!” No, seriously, that’s pretty gross, right? You’re probably wondering how rats could possibly be trained to go against such a powerful natural instinct. Well, here’s how:

Rats, humans, and all mammals have something in their brain called a “reward center.” [2] Part of the reward center’s job is to promote healthy living by rewarding you when you do something that either keeps you alive (e.g., eating) or creates a new life (e.g., sex), or enriches your life (e.g. building satisfying relationships). [3] The way it rewards you is by pumping a cocktail of “pleasure chemicals” through your brain. [4] (See How Porn Changes The Brain.)

Those chemicals do more than make you feel great. While you’re enjoying that good feeling, your brain is also building new nerve pathways to connect the pleasure you’re feeling to the activity you’re doing. [5] It’s the brain’s way of making sure that whatever you’re doing, you’ll come back to it again. The association between the activity and the “reward” happens automatically, even if you don’t intend it, because “neurons that fire together, wire together.” [6] (See How Porn Affects The Brain Like A Drug.)

The reward center is usually a pretty great thing, even if it didn’t work out so well for those poor rats. Normally our brain attracts us to healthy behaviors and encourages us to form life-supporting habits. [7] But when those reward chemicals get connected to something harmful, it has the opposite effect.

The same process that rewired those rats’ preferences—connecting the pleasure they felt during sex to the stench of death—is triggered in human brains by porn. Porn consumers may think they’re just being entertained, but their brains are busy at work building connections between their feelings of arousal and whatever’s happening on their screen. [8] And since consumers of porn typically become accustomed to the porn they’ve already seen and have to constantly move on to more extreme forms of pornography to get aroused, [9] the kind of porn consumed usually changes over time. [10] (See Why Consuming Porn Is An Escalating Behavior.)

In a survey of 1,500 young adult men, 56% said their tastes in porn had become “increasingly extreme or deviant.” [11] Just like the rats, many porn consumers eventually find themselves getting aroused by things that used to disgust them or things that they might have previously considered to be inappropriate or unethical. [12] In many cases, porn consumers find their tastes so changed that they can no longer respond sexually to their actual partners, though they can still respond to porn. [13]

Once consumers start viewing extreme and dangerous sex acts, things that they thought were disgusting or degrading can start to seem normal, acceptable, and more common than they really are. [14] One study found that people exposed to significant amounts of porn thought things like sex with animals and violent sex were twice as common as what those not exposed to porn thought. [15] And when people believe a behavior is normal, they’re more likely to try it. [16]

Research has also found that watching pornography affects attitudes and beliefs toward sex, women, and relationships. [17] Porn consumers are more likely to express attitudes supporting violence against women, [18] and studies have shown a strong correlation between men’s porn consumption and their likelihood to victimize women. [19] In fact, a 2015 peer-reviewed research study that analyzed 22 different studies from 7 different countries concluded that there is “little doubt that, on the average, individuals who consume pornography more frequently are more likely to hold attitudes [supporting] sexual aggression and engage in actual acts of sexual aggression.” [20] (See How Consuming Porn Can Lead to Violence.)

Obviously, not everyone who looks at porn is going to turn into a rapist, but the reality is that even casual pornography consumption has the power to change ideas and attitudes. [21] When that happens, changes to behavior aren’t far behind. But spreading the truth about the harmful effects of porn helps limit its influence. Porn can corrupt our deepest, most basic instincts, but deep down at that same instinctive level, we know and want what’s healthy. We crave happiness and love. And every individual decision to focus on real love and real relationships moves us back toward the robust, natural lives we’re wired to pursue.