The sensual circuit _cannot_ adjust itself spontaneously between the two.
So what have you?
Child and parent intensely linked in adult love-sympathy and love-will, on the upper plane, and in the child, the deeper sensual centers aroused, but finding no correspondent, no objective, no polarized connection with another person.

There they are, the powerful centers of sex, acting spasmodically, without balance.

They must be polarized somehow.

So they are polarized to the active upper centers within the child, and you get an introvert.
This is how introversion begins.

The lower sexual centers are aroused.
They find no sympathy, no connection, no response from outside, no expression.