John Hagedorn, who has long been an expert witness in gang-related court cases, claims that what transpires in the trials of gang members is a far cry from what we would consider justice. In Gangs on Trial, he recounts his decades of experience to show how stereotypes are used against gang members on trial and why that is harmful. Hagedorn uses real-life stories to explain how implicit bias often replaces evidence and how the demonization of gang members undermines fairness. Moreover, a "them and us" mentality leads to snap judgments that ignore the complexity of gang life in America.

Gangs on Trial dispels myths about gangs and recommends tactics for lawyers, mitigation specialists, and expert witnesses as well as offering insights for jurors. Hagedorn describes how minds are subconsciously "primed" when a defendant is identified as a gang member, and discusses the "backfire effect," which occurs when jurors hear arguments that run counter to their beliefs. He also reveals how attributional errors, prejudice, and racism impact sentences of nonwhite defendants.

Hagedorn argues that dehumanization is the psychological foundation of mass incarceration. Gangs on Trial advocates for practical sentencing reforms and humanizing justice.