There is an old idea that you can judge a man by the company he keeps. No doubt, the knowledge of the company he has kept would make the judgment even more secure. Folklore and habit put on social experience responsibility for a great deal of what people mean by personality and character.
But the step from folklore to science is a long one and the precise description of people's effects on people, which is a significant part of what contemporary psychiatric research is about, stands very far from being complete. If you roll one billiard ball against another, the measurements of velocity, vectors and angles can be nicely ascertained; the resulting conclusions have a formidable exactness. It is not this way with people's effects on people and it may never be, but a significant start has been made.
One difficulty with human research is, of course, that humans have
Roland CG. Social Nutrition and Malnutrition. Arch Intern Med. 1970;126(2):198–204. doi:10.1001/archinte.1970.00310080004001