The Forbidden Experiment

The Forbidden Experiment has been used to describe the idea of bringing up a child in isolation, to see what, if anything, he or she acquired in the way of language, and possibly other attributes we associate with humankind. Much of the interest in feral children by the scientific community is because of what they can teach us that we cannot ethically learn by experimentation.

In general, however, there will never be — and there never should be — the opportunity to study neglect in humans with the rigor that can be applied in animal models.
Perry, Childhood Experience and the Expression of Genetic Potential.

The forbidden experiment according to Psammetichus (Psammetik)

Of course, as with anything else forbidden, somebody has to try it. There are various versions of the following legend, attributed to different historical figures, but the Psammetik story is the best known.

The Egyptians before the reign of Psammetichus used to think that of all races in the world they were the most ancient: Psammetichus, however. when he came to the throne, took it into his head to settle this question of priority, and ever since his time the Egyptians have believed that the Phrygians surpass them in antiquity and that they themselves come second. Psammetichus, finding that mere inquiry failed to reveal which was the original race of mankind, devised an ingenious method of determining the matter. He took at random, from an ordinary family, two newly born infants and gave them to a shepherd to be brought up among his flocks, under strict orders that no one should utter a word in their presence. They were to be kept by themselves in a lonely cottage, and the shepherd was to bring in goats from time to time. to see that the babies had enough milk to drink and to look after them in any other way that was necessary. All these arrangements were made by Psammetichus because he wished to find out what word the children would first utter, once they had grown out of their meaningless baby/talk. The plan succeeded; two years later the shepherd, who during that time had done everything he had been told to do, happened one day to open the door of the cottage and go in, when both children running up to him with hands outstretched, pronounced the word "becos". The first time this occurred the shepherd made no mention of it; but later, when he found that every time he visited the children to attend to their needs the same word was constantly repeated by them he informed his master. Psammetichus ordered the children to be brought to him, and when he himself heard them say "becos" he determined to find out to what language the word belonged. His inquiries revealed that it was the Phrygian word for "bread", and in consideration of this the Egyptians yielded their claims and admitted the superior antiquity of the Phrygians.
From The Histories by Herodotus, Book II (Penguin Classics, Harmondsworth, Middlesex, 1966, pp . 102-103) .

In 1211, Frederick II, Emperor of Germany, in an attempt to discover the natural "language of God," raised dozens of children in silence. God's preferred language never emerged; the children never spoke any language and all ultimately died in childhood (van Cleve, 1972).
Perry, Childhood Experience and the Expression of Genetic Potential.