Invasion of the Body Snatchers (1978)
More info i
Credit: MGM/UA

Science Behind the Fiction: The many ways we've all been body snatched

Contributed by
May 22, 2019, 1:30 PM EDT

The spooky notion of supernatural doubles has existed for much of our documented history, but it really took root in the 1950s. World War II had ended, in its wake came concerns about enemies hidden in our midst. McCarthyism was at a fever pitch and several stories involving alien invaders taking over the form of human beings were published in short succession.

Robert Heinlein's The Puppet Masters, Ray Bradbury's It Came from Outer Space, and Phillip K. Dick's The Father-thing helped solidify the idea of alien invaders who took over human bodies, but Jack Finney's 1954 story The Body Snatchers became the most iconic example.

First serialized in Colliers Magazine, The Body Snatchers would later be adapted to film. Several times. First in 1956, then 1978, 1993, and finally 2007. While body snatchers, as a narrative device, never caught on with quite the same gusto as their spiritual sibling, zombies, they have remained a part of the conversation with a new book or movie for just about every generation. Assimilate, from director John Murlowski, hopes to fill that slot for the modern age.

Most movie monsters are frightening because they might attack us. The ultimate, underlying fear is loss of life. With body snatchers, the fear is two-fold and more existential. The primary fear is not loss of life, but loss of self. The idea that who we are might be eroded away in favor of something else. The second fear is that those we care for, those we trust, can no longer be trusted. What if you could no longer be certain that the people around you were who they claimed to be?


Recognition, as with most mental processes, is more complex than it seems at first glance. The brain is completing multiple functions, all in concert, to confirm that the person in front of you is, in fact, someone you know. When any one of those processes malfunctions, problems abound.

When you encounter a familiar person, be it a spouse, sibling, parent, or acquaintance, your brain goes through two functions simultaneously. First, the vision centers take in facial features and movements. Second, the parts of the brain responsible for emotional processing kick in. Think of recognition as a handshake. The first hand comes in and says "this looks like your mother" and the second hand says "yes, I know my mother and have warm feelings toward her."

Capgras occurs when the second hand never shows up. Instead, your brain tells you that while the person before you looks and moves like someone you know, they most certainly are not that person. The common conclusion among sufferers of Capgras is that their loved one has been replaced by a duplicate.

Capgras is most common in those with a history of mental illness but can also manifest as a result of brain injury or illness. And even though the sufferer might logically know that their perception is irrational, the emotional feeling is so pervasive it often overpowers any other considerations. Moreover, because the belief is paranoid in its nature, any suggestion to the contrary can be met with resistance.

The belief that those around us might not be who they say they are is frightening enough, but what if your own identity could be compromised?

There are a number of earthly organisms intent on bending your mind to their own machinations.


Fungi hold an important position in our environment. They act not only as a food source but are useful in the breakdown of organic material. Many fungi live independently of other life forms but some have developed symbiotic or parasitic relationships with other plants and animals.

In fact, some fungi are both parasitic and symbiotic. Fungus of the order Homoptera creates vast structures over the surface of trees which house scale insects. These insects insert their proboscis into the surface of the trees to gather nectar and remain there the rest of their lives. The fungus bores into the body of the insect without killing it, and feeds. While those individuals who are parasitized are sacrificed, the rest of the colony is protected from predators by the fungal body.

Not all fungi are quite so accommodating.

Cordyceps, a specialized form of fungi, infect specific insect populations in order to complete their life cycle. It's believed that each species of Cordyceps has a specific insect population it infects, foregoing all others.

Once infected, the fungus spreads through the insect's body until it reaches the head, releasing chemicals which take over the nervous system. At this point, the individual is no longer in control of its actions. While it might look just the same as it always did to its friends and family (to whatever extent insects have these relationships) inside, it is wholly changed.

The insect is compelled to climb to high ground and bear down. At which point the fungus grows, punching through the body to release spores and start the cycle again. This insect has been, for all intents and purposes, body snatched.

This sort of relationship, terrifying as it is, isn't isolated to insects, or even to non-human animals. Certain parasites are known to impact human beings, changing our behavior without our even knowing.

In fact, it's believed that thirty percent of all living humans are infected with a brain parasite capable of altering behavior. Look to your left, now look to your right. One of you has been body snatched.

Toxoplasma gondii is a small protozoan present in cat feces. It has been shown to alter the behavior of infected rats such that they willingly walk into the jaws of preying cats. And, it impacts humans, too.

Many of those infected will show no symptoms and there are steps you can take to prevent infections. The parasite doesn't become active for several days. So, regularly changing your cat's litter box is recommended.

For those of us already infected, it's likely you'll experience little to no symptoms as the parasite enters a chronic dormant state, however, some studies have shown that infection is related to an increase in extroversion and aggressive or risk-taking behavior.

While many of us have long suspected that cats were hatching plans for world domination, something much smaller and far more common has the lead on that front.

Each of us is host to an untold number of microbes living in our gut. They exist in symbiosis with us. We provide for them an environment in which to live and a constant source of sustenance, and they help us with digestion and development. It's been suggested that without our microbiome, we may not have reached our current level of cognitive development.

What's less commonly known is the impact these microbes have on our behavior. Changes in our microbiome have the capacity to impact our behavior, ranging from stress response, cognitive function, and social interaction. And your microbiome can be altered by significant life changes, antibiotics, or drastic changes in diet. Something to consider the next time you go on a weeks-long fast food bender.

While we've spent all our time worrying about silent invasion from beyond the stars, we've been taken over by cats and cheeseburgers. In the end, there's likely nothing to fear, these are relationships we've been living with for thousands of years or more. Any mind control going on is just our base state and we might as well get comfortable with it. It's unlikely to change any time soon.