Uri Bronfenbrenner’s book The Two Worlds of Childhood. Children in the USA and the USSR (Moscow: Progress, 1976) the author tells about a project that later became known as a thirty-year experiment. It was about how to bring mentally retarded children who are kept in a special shelter into people, to ensure that they can live independently. The experiment consisted of many stages, but the most important and curious was the very first of them.
Each child was attached to a sort of fake surrogate mother; such mothers were mentally retarded women kept in the same shelter. Two years later, special measurements showed that the level of intelligence in children increased by an average of 20-30 points; at the same time, the level of intelligence in the children of the control group decreased. These mothers obviously could not conduct any kind of developmental activities with their children: no mathematical circles, no puzzles, no intellectual games. All they could do was hug children, kiss them, swaddle them, and in general make out with them in every possible way. And so, it turns out that, at least at a certain age, emotional warmth, parental affection is much more important for the development of the child, including for the development of his intellect, than any other form of activity and education! From A.K. Zvonkin «Kids and Mathematics. Home circle for a preschooler, download the book (pdf). You can read more about the book here, and a snippet of the book here.
At the same time, it is hardly true to say confidently that without parental love a child cannot grow up healthy and developed. See →