When Eric Harris arrived at Columbine High School on the morning of April 20, 1999, he was wearing a “Natural Selection” T-shirt. Before Finnish student Pekka-Eric Auvinen murdered eight people at his high school in November 2007, he wrote that “­stupid, weak-minded people are reproducing…faster than the ­intelligent, strong-minded” ones. “Death and killing is not a ­tragedy,” he went on, “it happens in nature all the time.”

Auvinen’s YouTube handle was “NaturalSelector89” and both boys, says Dennis Sewell, were “amateur social Darwinists”: they used evolutionary theory to justify their atrocities. In this polemical mini-history of the political abuses of Darwinism, Sewell shows how they were part of a miserably long tradition, taking in everything from forced sterilisation to mass murder. It is a disturbing and provocative book.

Sewell admits that Darwin himself was a man inclined to gentleness and modesty, but early enthusiasts for his theory could be a little redder in claw. In Britain, Darwin’s friend Herbert Spencer — who coined the term “survival of the fittest” — argued vociferously against state aid for the indigent. If people “are sufficiently complete to live, they do live”, he wrote; if not, “they die, and it is best they should die”. Or, as Darwin himself observed, “excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed”.

Fabians, socialists and all kinds of nefarious interventionists eagerly signed up to the cause. HG Wells complained that “we cannot make the social life and the world peace we are determined to make, with the ill-bred, ill-trained swarms of inferior citizens”. In America, campaigners paid homeless men to walk around wearing sandwich boards with the legend “I am a ­burden to myself and the state. Should I be allowed to propagate?”

Eugenics, as the movement became known, was soon sanctioned by law. Before it was abolished in 1959, England’s Mental Deficiency Act of 1913 divided the unfit into “idiots, imbeciles, the feeble-minded and moral defectives”, and segregated them in “colonies”. Sewell tells tales of monstrous ­injustice, such as the “depraved and hopeless little animal” who, following a lobotomy in an asylum at 14, became “quite a sociable, clean child”. “The Darwinists,” Sewell says, “had truly taken over the asylum.”

In some rural areas of America, sweep teams would set up roadblocks and break down doors in search of defectives. But in 1927 a test case arose around the legal treatment of Carrie Buck, who became pregnant after being raped by her foster parents’ nephew. Like her mother before her, she was selected for compulsory sterilisation and committed to a ­Colony for the Feeble-minded; her seven-month-old baby was declared to have “a look about it that is not quite normal” — and handed back to the same foster family. In his 1927 Supreme Court judgment on the case, ­Justice Oliver Wendell Holmes Jr ­concluded that “three generations of imbeciles are enough”. Compulsory sterilisation laws remained in place in some states until the 1970s, and in Oregon until 1983.

Eugenicist ideology reached its European apogee with Nazism. Sewell approvingly quotes David Klinghoffer, the essayist who blames Auschwitz on “moral relativism” and a belief that “violent competition in nature creates greater and lesser races”. Curiously, he fails to mention that Klinghoffer is a leading figure in the religious-based anti-Darwinist “intelligent design” movement. More worryingly, Sewell only cursorily admits that Christian anti-semitism may have played its part in inspiring Nazi race hatred.

Sewell is no proponent of intelligent design. He has “no quarrel whatsoever with the orthodox account of evolution as taught in English schools”, he says. But he does have an agenda. He dredges up the usual mud to fling at Marie Stopes for her various eccentric eugenicist beliefs, links the British 1960s campaign for abortion-law reform to the eugenics movement, and gives the last word on the ­subject of family planning to Pope Benedict XVI.

His chief concern, however, is that a militant, unrepentant “Darwinist/atheist axis” is trying to reignite the eugenicist candle. If it’s not dodgy research into racial differences in IQ, he says, it’s the miscreation of green, fluorescent, transgenic macaques. On the evidence he presents, he is probably right that geneticists “are no more ready to take the controls of human evolution than a two-year-old infant is to fly a high-performance jet fighter”. But when he wonders “whether perhaps too much exposure to ­evolutionary science might itself lead to a degeneration of the faculty of moral discernment”, he surely goes too far. The problem with ­evolution isn’t the theory, it’s when someone tries to put it into practice. Eric Harris of Columbine also wrote that he felt like God, after all.

The Political Gene by Dennis Sewell
Picador £16.99 pp288

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