A-humorous-cartoon-depicting-two-diverse-identical-twins-having-a-dramatic-mountaintop-showdown.-One-twin-is-dressed-in-light-heroic-clothing-with-a-white cape, the other is in dark colours

The Evil Twin Hypothesis

As everyone knows, one twin is always evil. It’s such a common theme in literature and popular media that it has to be true, doesn’t it? Whenever there are twins, one of them is likely to be evil. But why is this? Myself and a colleague have discussed the science and believe that we have part of the answer. Bizarrely, we can tell you why one from a pair of two identical twins will be evil. If the twins are non-identical then our argument doesn’t work. In fact, I don’t even know if the evil twin law even applies for non-identical twins. Possibly not.

Why should this be? Surely, identical twins are, well… identical? Superficially, that is how it seems to outsiders. But all the evidence is that if two people look identical, then, however nice they both seem at first, sooner or later one of them will turn out to be mad, crazy or just plain evil. The key is little differences at the start of life make a big difference at that mountain-top showdown.  It’s all to do with sibling rivalry and it all begins in the womb.

Life begins against the odds. A single egg is fertilised by single sperm, one out of millions. A zygote is created. Sadly most of the time, life never gets any further than that.. the majority of zygotes (60%) never implant on the wall of the womb. When one does it has some chance of developing into an embryo. About 1/5 of the time there will be a spontaneous miscarriage resulting in no baby but in 3 out of 1000 cases the cell splits in two, and there are two separate embryos and two babies, identical twins.

Genetically identical twins are physically different individuals. They have different life histories and so although they seem the same, there are important differences. This is obvious to the twins themselves but not always to everyone else. And this is where the problems arise. One twin always knows that despite appearances they’ve had an unfair deal and this is what turns them evil.

It is due to those first nine months in the womb. When a single cell splits and two embryo in the place of one, there is inevitably competition. Both competing for the resources of the mother. And as the embryos and their placentas grow then slowly but inevitably one of them gets a bigger share. Two babies are born, they started with identical genetic material but that little difference is the beginning of a lifelong rivalry.

Once the twins are born, the disparity that began in the womb becomes more pronounced. It starts small: one twin might get the slightly fluffier blanket or the rattle that makes a more pleasing sound. These differences, trivial as they seem to others, are strongly felt by the twins. Tiny differences lay the foundation for a burgeoning sense of injustice. The ‘evil’ twin begins to harbor a deep-seated resentment, fueled by every perceived slight and unfair distribution of parental affection.

As they grow, the world continues to conspire against them. The so-called ‘good’ twin always seems to get the first piece of birthday cake, the front seat in the car, and the lion’s share of compliments from relatives. Meanwhile, the ‘evil’ twin is left with the second-rate, the leftovers, and the politely indifferent “Oh, and you too” acknowledgments. It’s enough to drive anyone to a life of nefarious deeds and sinister plotting.

By the time adolescence hits, the ‘evil’ twin has fully embraced their role. They’ve learned to weaponize their grievances, mastering the art of the passive-aggressive comment and the withering glare. While their twin is off winning sports trophies or acing exams, the ‘evil’ twin is busy honing skills in sabotage and psychological warfare.

This isn’t to say that the ‘good’ twin is entirely blameless. In their quest to be the perfect child, they inadvertently fuel their twin’s villainous transformation. Every gold star on a homework assignment, every enthusiastic cheer at a soccer game, only serves to deepen the ‘evil’ twin’s resolve to one day emerge victorious in their ultimate showdown.

Of course, the inevitable climax of this lifelong drama is the classic mountaintop duel, complete with dramatic lightning and a soundtrack by a full orchestra. The twins, now fully grown, face each other, one radiating a noble glow, the other shrouded in ominous shadows. It’s here, in the thin air and the eerie silence of the peak, that the final battle takes place. The fight is less about physical prowess and more about who can deliver the most cutting remark or the most guilt-inducing revelation.

In the end, it doesn’t really matter who wins the duel. The true victory is in the journey—the decades of scheming, plotting, and counter-plotting that make life with an evil twin so delightfully unpredictable. And so, the legend of the evil twin persists, a cautionary tale and an entertainment staple, all because two embryos couldn’t play nice in the womb.

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