An image says more than a thousand words. And in this case the image is a brain scan... or rather two side by side:
|Both brains belong to 3-years-old children|
... the child with the shrunken brain was neglected and abused by its mother, and the child with the larger and more fully developed brain was raised in a loving, supportive home and was looked after by its mother...
This is not just about IQ or head size, which may vary at least largely because of this type of environmental causes, but about everything in life, including emotional and social intelligence and the general ability to carry on with a normal, well integrated life (or become human waste).
The first years of life are critical for all our development and parental love, very specially that of our mothers, is probably more important than almost anything else.
Source: Medical Daily.