Dalrymple writes in Psychology Today on a decades-old study the methodology of which is very troubling:

The lecturer reported an experiment in which children suffering from the syndrome were divided into three groups after they had been separated from their mothers and placed in an institution: the first was given attention and adequate food, the second adequate food but no attention, and the third attention but inadequate food. Those who were given adequate food grew quickly, but those who were not given such food failed to do so; attention added no extra growth. Therefore, the dwarfism of maternal deprivation syndrome was caused by poor nutrition.