Why Kids Write Their Letters Backwards

Humans have been on Earth for around 200,000 years but have only used symbols as a form of communication for a fraction of that time. Because of this, Mariano Sigman, author of "The Secret Life of The Mind: How Your Brain Thinks, Feels and Decides," explains why children must unlearn the natural way in which our brains recognize images and symbols.
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Following is the transcript of the video:
Mariano Sigman: We, and by we I mean modern humans, have been here on Earth for about 200,000 years. But only about 5,000 years ago, we discovered that we could convey what we were thinking using visual symbols. And this actually has been an incredible source for human cultural explosion. Suddenly, the thoughts we had could be written in a stone, later in a paper, but somewhere where it could be kept for generations and generations. This has been a major transformation of human culture. Now, this poses a very interesting puzzle for the biology of reading because 5,000 years are not long enough, to evolve or to change our brains for the capacity of reading. When we understand that, we also understand that there ought to be many ways on how the biological machinery for reading, are actually set from other domains of vision.
One of them, a very fundamental one, is that vision is essentially invariant rotations. So this is my hand, but this is also my hand, and this is also my hand, and this is also my hand. And this is true, essentially for all the objects we deal with. Now there is an exception for that, which is letters. If you get a "p" and you rotate it, it's a "q," and if you rotate it like this it's a "b" and then a "d." Actually letters are not rotation invariant. We've chosen visual symbols, it's just been the choice of our cultures that actually they do not have this property that the visual system outside of reading has.
So this explains something that actually seems very strange, very weird, but actually it's very natural, very simple, and quite extraordinary. Only parents know that when children start to write, they begin doing so writing words, so to say, normally in the same sense and orientation that we write them. But also spontaneously, when they begin to write, they will write the mirror image of the same words.
Now again, this often is something that remains unnoticed, sometimes some parents will say, oh that's kind of weird, or that's kind of strange, but at the same time we have to think that it's extraordinary, because it's something we cannot do. If an adult would like to do as children do, just to write the mirror image of the words, we would see that it's extremely difficult, we cannot do it, it takes a lot of effort. So how come children so young, they are doing something which we adults cannot even think of doing? And the reason is that they're using, for letters, the visual system as we use the visual system for all other things. Understanding that an image and the mirror rotation or reflection of that image corresponds to the same thing. This is how vision always works.
So one important thing of learning to write, is unlearning this property, that the reflection of on object corresponds to the same object. This is a very specific property of writing and children, through the process of learning, need to unlearn this thing that has been a regularity of the entire evolution of human vision.