Two films this week deal with cults - one is based on going back to nature, the other on the cleansing power of blood and bullets.

The first few minutes of Captain Fantastic(M18, 119 minutes, opens tomorrow, 4.5/5 stars) see a chase through thick forest - a deer is run down, killed and butchered by a bearded man and knife-wielding children.

It's a scene that gives the impression that this movie is a grim account of life in a survivalist camp or a drama about a conspiracy-nut father raising a generation of lunatic fringers.

Then the jokes show up - sly, understated flashes of humour showing the kids are all right. More than all right, actually. All six are emotionally grounded and wise beyond their years, quite often wiser than their idealist dad.

Ben Cash (Viggo Mortensen) and his brood have set up home in the leafy splendour of the forests of the Pacific Northwest. No telephones, computers or screens of any kind distract the kids from the regime of hunting-gathering, meditation, strenuous exercise and readings from the classics of the American Left. Instead of Christmas - an orgy of consumerism, thinks Ben - the family celebrate Noam Chomsky's birthday.

"My children will be philosopher- kings," declares Ben, and, by the looks of it, he seems to have succeeded.

After a decade of isolation, a family emergency forces them to leave their refuge and in a schoolbus named Steve, the group smacks headfirst into fast-food, high- fructose-corn-syrup America.

Filled with memorable characters and winning performances - especially from the children - this work bears a resemblance to one version of the Sundance dramedy, also known as the Little Miss Sunshine template, after the 2006 work: A rambunctious crew embark on a road trip, "normal" onlookers are baffled by their familial eccentricities, while crusty seniors and adorable moppets attack the delusions of the middle generation from both sides.

So the festival-circuit dramedy has been formularised and, indeed, this film was screened at the Sundance Film Festival and bagged the directing prize at the Un Certain Regard section at the Cannes International Film Festival.

But writer-director Matt Ross has such genuine affection for his characters that even the most cynical inner voice will be quietened.

Ross knows when to dial up the whimsy, in the artsy-craftsy, thrift-store fashion the family prefers, for instance. But he pulls it back before it starts to cloy. Animals have to die, so the family can be fed. Ben delivers a clinical description of human reproduction when his young children ask about sex. It is wildly inappropriate, but comprehensible within Ben's moral framework.

The director is also a long-time character actor, best known for playing Gavin Belson, the Type A boss of a tech firm in the HBO series Silicon Valley (2014- present). Knowing what actors can do is perhaps why he prefers that here, facial expressions, often flickering for a fraction of a second, tell the story as much as the dialogue does.

Mortensen excels as Ben, a character who, if cast wrongly, might have come across as abusive (as other characters allege) or worse, a fundamentalist looney intent on raising Unabombers. Mortensen imbues Ben with such an easy, thoughtful, virile confidence that it is easy to see why his wife Leslie (Trin Miller) and children would think that hiding in the forest with him seems like the best possible option.

By standard movie reckoning, the low-budget action-thriller The Purge: Election Year(NC16, 109 minutes, opens tomorrow, 3.5/5 stars), the third in the Purge series, should be a lazy rehashing of old ideas, dressed up with louder bangs and stunts.

While it's still a long way from being original, writer-director James DeMonaco has the good sense and integrity to not repeat himself, making the third movie much more entertaining than one would reasonably expect.

Based on the premise that a future America will fix its crime problem by making one night of the year free of police intervention and legal consequences, the first movie, The Purge (2013), was home invasion horror. The second, The Purge: Anarchy (2014), was a western, with bands of survivors traversing the city's hostile streets.

Now, it's become a political thriller with a high body count, with Beneath The Planet Of The Apes (1970) doomsday cult overtones.

Some years after the second movie, cop Leo Barnes (Frank Grillo) is now assigned to protect presidential candidate Charlie Roan (Elizabeth Mitchell), a woman who vows to repeal Purge Night if elected. When the Night rolls around, she becomes the target of a politico-religious conspiracy bent on keeping the tradition alive.

Mitchell and Barnes are on the run and get help from a rag-tag group, among them shopkeeper Joe (Mykelti Williamson) and undocumented migrant Marcos (Joseph Julian Soria).

DeMonaco loves 1970s B-movies, and this work is replete with over-the-top performances and idiots throwing themselves in front of bullets and chainsaws while dressed in ridiculously flimsy Halloween clothes. You would think at the very least they would don paintball gear. Some people just prefer to combine cosplay with gunplay.

What's also apparent is DeMonaco's sympathies. He seems to draw a line from Trump's right-wing bias to Purge Night, which exists because elites have brainwashed the poor into believing that self-harm is self-help, and that if a race and class war is inevitable, it's better to let it happen in a controlled burst of violence before it builds into a full-scale conflict.

Given what's happened in recent weeks in America, DeMonaco's fiction comes uncomfortably close to fact.