control group, those who sniffed oxytocin perceived in-group members as more likeable, more human, more richly endowed with social emotions such as embarrassment, contempt, humiliation and admiration, and more worthy of saving in an emergency. Oxytocin increases our sense of camaraderie toward those within the inner sanctum, which can result in greater animosity toward those outside. Oxytocin may therefore facilitate our ability to take out the competition even if this means killing another human being. Oxytocin is two-faced, cuddling with its left profile and harming with its right. This is a small sampling of the ways in which our promiscuous brain enables new forms of harm, including killing other adults. We didn’t invent lethal aggression. We share this capacity with a small group of animals that also kill other adults. But whereas these other species typically restrict their lethal attacks to situations in which one group greatly outnumbers another, typically targeting adults from a neighboring group, we evolved far beyond this monogamous approach. We adopted the cost-benefit analysis that drives killing in other animals and applied it to killing in a virtually limitless space of homicidal opportunities. We kill when we outnumber our opponents or are outnumbered by them, attacking individuals within and outside our core group. We kill spouses, ex-lovers, stepchildren, those who believe in God and those who don’t, the wealthy and the poor, kin and non-kin, and even ourselves if the cause is good enough. Virtually anything goes. Our promiscuous brains opened a Pandora’s box of harmful means, including the capacity to address a multitude of injustices. This is a capacity that is inherently good, but incidentally bad. It is a capacity that evolved in response to growing pressures to balance inequities and take care of those who attempt to cheat society. It is a capacity that enabled us to engage in punishment in a broad range of contexts, righting wrongs and open