Despite being arguably the most famous psychological experiment in history, the Stanford Prison Experiment – the one where volunteers were assigned to play either inmates or guards in a simulated jail, with ugly results – hasn't been explored much on screen. Several TV shows (Veronica Mars and Castle among them) have used it as a one-episode plot device, while last year's movie was a workmanlike adaptation of the ill-fated 1971 study, which was aborted after just six days after the "guards" became dangerously abusive.

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Orange is the New Black is too subtle to call out the study by name, but its conclusion about the corrupting influence of authority looms large throughout the disturbing fourth season, as Litchfield's inmates are terrorised by a new regime of unsupervised guards drunk on their own power. With Caputo now preoccupied with privatisation politics, the new man in charge is maximum-security officer Desi Piscatella (Brad William Henke), whose emphasis on order and protocol barely masks his sociopathy. And in the season's brutal, brilliant twelfth episode, 'The Animals', the sadistic power play spiralled into tragedy.

The signposting isn't subtle – nearing the end of her sentence, Samira Wiley's intensely lovable Poussey is starting to make plans for her future with new girlfriend Soso. And is even offered a job by celebrity chef Judy King. In TV terms, this is almost as sure a sign of impending doom as an ominous cough, but even with all the foreshadowing you don't see the gut punch coming. As the inmates finally take a (literal) stand-in protest, the guards resort to force to get them moving, and in the chaos Poussey is suffocated. Her death is excruciating to watch, very deliberately evoking the real-life death of Eric Garner, whose cry of "I can't breathe" has become a rallying cry for the Black Lives Matter movement.

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This isn't a show that kills characters lightly, much less characters as beloved and essential as Poussey, and it's hard to imagine how it can ever return to the comedic tone of earlier seasons. Almost as surprising as her death in itself was the fact that she was killed not by Piscatella or one of his recruits, but by Officer Bayley, who's a holdover from last season and the only decent guard in the bunch. 'Animals' is a flashback episode for Bayley, which feels like a strange choice until Poussey's backstory unfolds directly afterwards in the season finale, and you realise their backstories are almost identical. Both were arrested for trespassing and drug possession, but Bayley got a slap on the wrist and Poussey got six years in jail. It's one of the most subtle comments on racial inequality in a season full of them.

Even before the horrifying climax, season four is uncomfortable to watch in a way that OITNB hasn't been before – take Pennsatucky bonding with her rapist, or Caputo's spineless obedience to his new squeeze Linda, the gun-toting embodiment of casual corporate evil. But the increased darkness also makes for increased tenderness in relationships like Piper and Alex's romance, Maritza and Flaca's friendship, and Red and Nicky's mother-daughter bond.

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One of the season's greatest triumphs is its exploration of mental illness both through Suzanne and through Lolly, whose paranoid psychosis has been played in the past as dark comedy. Her relationship with the prison counsellor Sam Healy is an unexpectedly heartbreaking one – Healy still being, along with Pennsatucky, the greatest example of OITNB's ability to humanise characters who once seemed irredeemable.

Orange is the New Black will be a fundamentally different show from this point onwards. The brutality of this season speaks to the writers' refusal to take the easy path, or to coast on their established success, and that makes the show thrilling to watch even in its darkest, more desperate moments.

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