When the human being enters his new physical body, something arises which is analogous to his experience on discarding this body at the moment of death. When he enters his new physical body, the human being has a kind of fore-vision of his coming life, even as at the moment of death he has a retrospection of his past life. But he forgets this fore-vision, because the constitution of his physical body does net yet allow him to retain it in his memory.
At this moment the human being realises: “These are the family-conditions into which I am born, these are the geographical and local conditions and my destiny …” And at that moment it may sometimes occur that when the human being thus foresees a sad or a terrible experience which lies in store for him, he receives a shock and is afraid of the life which awaits him, so that his etheric body does not properly unite with the physical body, it does not wish to enter it.
Idiocy is the result of such a fright of the etheric body’s reluctance to enter properly into the physical body. A clairvoyant may perceive the etheric body of such people protruding above the physical head and because the etheric body is not properly structured into the physical head, the brain remains behind in its development, for the etheric body does not work upon it as it should. Many cases of idiocy today are dependent upon this.
Source: Rudolf Steiner – GA 100 – Theosophy and Rosicrucianism: Lecture VIII: Supplementary Thoughts on the Law of Reincarnation and Karma – Kassel, 23rd June 1907