"In any dispute, the intensity of feeling is inversely proportional to the value of the issues at stake." That's Sayre's law, credited to the late Wallace Stanley Sayre, a political scientist and professor at Columbia University. In a perfect irony, the precise origins of Sayre's law are themselves under dispute, but it's often applied to the halls of academia, where petty disagreements can fester into explosive personal vendettas, with little real-world import whatsoever. Anyone who's been to a condo meeting can probably tell stories about some throwdown over external vents in the common façade or, say, the Great Tuckpointing War of '08.

The Icelandic black comedy Under the Tree takes Sayre's law to its natural conclusion. In fact, the word "comedy" doesn't really apply to Hafsteinn Gunnar Sigurðsson's mirthless dissection of neighborly discord; the laughs are strictly of the stuck-in-the-throat variety, tied to the acknowledgement of grim ironies or absurd truths about human behavior. Here, a fight over an untrimmed tree exposes the roots of deeper problems within each home, including troubled marriages, an unexplained death in the family, and simmering jealousies and resentments. Sigurðsson is attuned to how conflicts can escalate from the tiniest infraction and he sees this one through to a conclusion that's as shocking as it is coldly logical.

Sigurðsson's first look at the tree in question is like a God's-eye-view from a Terrance Malick film, but for the neighbors, those heavenly sun-dappled leaves are too much dapple, not enough sun. Konrad (Þorsteinn Bachmann) and Eybjorg (Selma Björnsdóttir) have repeatedly asked the older couple next door, Baldvin (Sigurður Sigurjónsson) and Inga (Edda Björgvinsdóttir), to trim the tree, because the shade swallows up their back porch and keeps Eybjorg from sunbathing. Inga rolls her eyes at Eybjorg's vanity, and the two husbands mostly serve as dutiful footsoldiers in the passive-aggressive — and, later, aggressive-aggressive — war between their wives.

Yet there's more context to the case than that. The true center of Under the Tree is Baldvin and Inga's grown son Atli (Steinþór Hróar Steinþórsson), who has moved back home after his wife Agnes (Lára Jóhanna Jónsdóttir) kicked him out. And for good reason: In the opening scene, Agnes catches a sexually frustrated Atli on his laptop, watching an old sex tape he and his ex-girlfriend had put together. Atli's subsequent efforts to reconcile with his wife and kindergarten-age daughter backfire horribly, as he stalks Agnes at work and condo-board meetings and yanks his kid out of school for an impromptu picnic. At least some of his and his parents' duress is explained by the mysterious "disappearance" of his brother, which Atli understands to be a suicide while his mother walls herself up in denial.

Hilarious, right? Sigurðsson does emphasize the petty absurdity of the tree issue and how it brings out the worst of everyone involved, but the retaliatory measures grow so extreme that only the most black-hearted will be inclined to laugh. (There are house pets in play, if that's any indication.) Once it's clear, however, that Under the Tree is fully committed to seeing its premise through to the darkest conclusion, it does drain the film of surprise. There are a couple of Chekhov's guns that are destined to go off and a twist in the denouement that could be predicted from about 45 minutes earlier. Sigurðsson sets up certain inevitabilities that ratchet up the dread while losing a little spontaneity.

Under the Tree should be respected for going all the way, though, and for treating a comic situation with emotional realism. These are not bourgeois cartoons doing battle, but broken people lashing out, especially Atli and Inga, who are still smarting from pains much deeper than sloppy landscaping. One of the most unsettling aspects of the film is how difficult it becomes to categorize it, because "black comedy" doesn't fully cover the serious domestic frisson that sends this story down its inexorable course. Under the Tree may be a demonstration of Sayre's law, but Sigurðsson doesn't reduce his characters to faceless suburbanites, ripe for the skewering. Their misdeeds are chillingly relatable.

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