An individual finding themselves in a totally different body is a familiar occurrence in fiction, one which usually results in various wacky antics. But according to current scientific understanding, if such a thing were even possible, the consequences would likely be quite traumatic

Next year sees the release of the Ben Kingsley/Ryan Reynolds film Selfless, where a wealthy elderly dying man transfers his consciousness from his diseased and ravaged body into a younger, sexier one. This is quite a familiar trope in fiction, the body swap, where a character finds his or herself inhabiting a completely different body.

Sometimes it’s an individual ending up in an unfamiliar form, other times it’s two people known to each other who somehow end up swapping bodies. One of the earliest known examples of the body swap is the 19th century novel Vice Versa, but it’s been used often since. As well as mainstream comedies like Freaky Friday or The Change Up where it’s essentially a comedic device, there are variations of it in the literature, some of which try to explain the process without using magic. Richard Morgan’s brilliant (but brutal) cyberpunk Altered Carbon series describes a future where consciousness has been digitised, meaning it can be stored as you would a computer file. This leads to minds being transferred between bodies like operating systems between computers.

Alternatively, there’s David Brin’s Kiln People (one of my favourite ever sci-fi books) which invents a whole new field of science to explain it. In this world the mind can be copied and transferred into duplicates, but never swapped between humans.

If there is any science behind body swapping, we don’t know what it is, so don’t know how it would be done. A “brain transplant” would be less than ideal as it would be indescribably difficult; even if you could reconnect the millions of neurones between brain and spinal cord, nerve cells don’t regenerate very well. And this is assuming the eyes and optic nerves are transferred along with the brain, because the eyes are anatomically part of the brain, and the connection between eyes and visual cortex are just as if not more frighteningly complex as those between brain and spine.

But most fictional body swaps don’t invoke brain transplants. It’s more a swapping of minds, suggesting a dualistic philosophy, where mind and brain are separate things. This is in contrast to a monistic stance (mind is created by the brain) which most relevant scientists would likely adhere to, given that a separate mind is unobservable as far as we know, ergo scientific analysis of it is impossible.

But let’s just assume that the mind can be separated from the body and, by whatever means, transferred into a different one, meaning the body is now inhabited by a new consciousness. That fact that the mind needs to inhabit a body suggests a Cartesian dualist set-up, where mind and body are separate but interdependent, so the mind still needs to use the brain. Ergo, there would still be a lot of issues to contend with. Exactly what they are depends on the type of body swap. There are several possibilities.

Body-type to same body-type swap.

Even if the bodies being swapped are relatively similar, i.e. 30+ Caucasian male swapping bodies with 30+ Caucasian male like in The Change Up , there would still be problems. Primarily, the mind now occupies a brain it hasn’t developed alongside/from, so the brain has a lot of features that will be unfamiliar. For example, memories are physically stored in the brain. Even if the newly transferred mind brings its own memories with it, now there are 2 sets of memories in one head (one physical, one purely mental)? That’s going to be disorientating to say the least.

But even if the new mind’s memories somehow “overwrite” the original ones, the brain architecture itself is going to be different. The brain changes over time depending on our experiences. A musician is going to have a subtly different brain to that of a taxi driver; both occupations require completely different skill sets, and the brain layout will reflect this. So if a musician’s mind were to be inserted into a taxi-driver’s body, he would likely find himself without his usual skills, and with an enlarged and active area of his brain dedicated to driving and navigation that he has no idea what to do with. And unregulated brain activity doesn’t usually bode well.

Even if neither body has any such obvious skill, the brain maps the body very precisely via the cortical homunculus, so if you’re in a different body then you’re going to have a differently configured cortical homunculus, which could only prove debilitating at first. It would be like wearing glasses with the wrong prescription, only inside your head and substantially worse.

Body swapping into a different sex.

There’s nothing more hilarious than when someone suddenly ends up in a body that is a different sex to their usual one. Of course, sex changes and the fluidity of gender identity are real things, but I’m not going to get into those here; it’s a very sensitive subject, and attempting to explain it would be the online equivalent of playing hopscotch in a minefield. So let’s stick to the Hollywood version, which usually involves a ridiculously stereotypical man swapping bodies with an equally stereotypical woman.

Largely, all the issues in the previous section still apply; men and women don’t have markedly different brains, despite what you may have heard. There are still some differences though, and these can’t be ignored. As well as a completely different and neurologically complex reproductive system to contend with, the different hormonal balance of the new body would be something the mind would have no experience dealing with, and hormones affect cognition and emotions among other things.

Even if you were somehow able to cope with the different body and the myriad issues that would cause, the surge of unfamiliar hormones you’d have to regularly deal with would be like going through puberty again. Speaking of which...

Child to adult/adult to child body swap.

A child in an adult body; given that this is what happens in Big and 13 going on 30 (plus arguably every Adam Sandler film) this is perhaps one of the better known examples of body swapping. You could argue that it should be the most consequence free, as quite often it’s the child entering an older version of their own body. But this could be the most disastrous of all. A lot happens during adolescence that is integral to our psychological development (we learn how to evaluate risks, find boundaries, gain independence, deal with our sexual drives, and so on), and a child’s brain is markedly different from an adults.

Child brains have a lot more connections than adult brains, clearing these is an important part of adolescent development. So a child suddenly finds themselves thrust into a brain which is more rigid, qualitatively different and subject to constant experiences and drives that they have no idea how to comprehend or process. Nobody could blame them if they got upset and started lashing out. Pity they’re in a fully grown adult form where they can do serious damage.

At least an adult ending up an a child’s body would have some experience at dealing with it, but the now-unfamiliar sensations and mental processing would still take some getting used to. Plus, they’d be a lot smaller all of a sudden, and that would be annoying as much as anything.

Human to animal body swap

Honestly, I’ve no idea how this would pan out. But please enjoy the accompanying picture for this article, which is an artist’s impression of me trying to find out. For science!

Dean Burnett has probably earned many Guardian points for managing to reference 13 going on 30 and Cartesian Dualism in the same article. Feel free to mock him for this on Twitter, @garwboy