... "social background .....
I'll bet that the first two words in this verbal trio tripped you up: I'll bet you thought you were going to read about background evidence with a social flavor. But in fact when you encounter this label -- "rotten social background evidence" -- in a criminal trial, you are more likely to encounter evidence about a person's (rotten) social background -- and such evidence about background may not be background evidence 'tall. See generally Mythri A. Jayaraman, "Rotten Social Background Revisited," 14 Cap. Def. J. 327 (2002).
P.S. The pedant in me thought you might want to know.
P.P.S. Isn't the title of Jayaraman's article delicious?
P.P.P.S. And don't you think it's time for the publication of a legal treatise with the title Rotten Social Background Evidence? The book would be a best seller -- if, at least, there were a pinch of humor in the tome. And the book could not fairly fail in the scholarly community either: the author could always say (justly), "But I promised you rotten evidence. And I delivered. What's your beef? It takes skill and intelligence to craft consistent nonsense." And you, the expert in criminal evidence (or a similarly-nomenclatured field) might only be able to respond with a splutter -- unless, that is, you are juvenile enough (sufficiently open-minded?) to be surprised (and perturbed) by the law's verbal quirks. Criminal law indeed!