Spectator effects represent a central concept in (behavioral) sports economics. A thorough understanding of the phenomenon promises to further our understanding as to the nature of performance production under pressure. In traditional home advantage studies, it is difficult to isolate the net crowd effect upon relative team performance. In a typical sports setting, multiple factors change at once for a visiting team. Experimental evidence suggests that supportive crowds may hinder task performance. In that it serves as home stadium to two National Basketball Association teams, the Staples Center in Los Angeles offers a rare natural experiment through which to isolate the crowd effect upon competitive output. Each team possesses equivalent familiarity with built environment, and teams face similarly sparse travel demands prior to games between one another. However, the team designated as "home team" in a contest enjoys a largely sympathetic crowd due primarily to season ticket sales. Moreover, crowd effects are sizable in motivating a home team win, raising the likelihood of such an event by between an estimated 21 and 22.8 percentage points. The point estimate implies that essentially the entire home advantage between the two teams is attributable to the crowd effect.