From ExchangeEveryDay :

« Having had a difficult relationship with one’s parents is a greater risk factor for adult disease than smoking, obesity, and high blood pressure combined. » This dramatic claim was made by John Robbins in his best-selling book, Healthy at 100 (New York: Ballantine Books, 2007). To illustrate his point Robbins describes a study launched in the 1950s by Harvard University in which 125 randomly selected undergraduate students were quizzed about their relationships with their parents. 

« The students were asked, ‘What kind of person is your mother?’ and ‘What kind of person is your father?’ The researchers simply counted the number of positive and negative words the students wrote down in describing their parents…And it was immensely powerful. Fully 95 percent of the students who had used few positive words to describe their parents developed serious diseases in mid-life, whereas only 29 percent of those who had used many positive words developed comparable diseases. »

Robbins concludes by quoting surgeon Bernie Siegel who observed, « The greatest disease of mankind is lack of love for children. »