What is the proper subject of expert testimony in a criminal case?

The expert testimony regarding the custom and practice of Avenues gang members, who used souvenir miniature bats as weapons against rival gang members, was the proper subject of expert testimony. In People v. Olguin (1994) 31 Cal.App.4th 1355, the reviewing court affirmed the admission of gang expert testimony, based on his interviews with gang members and reviews of police reports, to his opinion that the Southside gang would retaliate violently if its graffiti had been crossed out. (Id. at pp. 1370-1371.)