Rourke continued, “The structure in question probably began to evolve toward its modern human form in the primates, though it had precursors in earlier mammals. It was first identified and studied – in chimpanzees – by a neuroscientist called Lamont, in 2014. The corresponding human version was mapped a few years later.

“Maybe the first crucial role for Lamont’s area was to help make deception possible – to learn how to hide your own true motives, by understanding how others perceive you. If you know how to appear to be servile or cooperative – whatever’s really on your mind – you have a better chance of stealing food, or a quick fuck with someone else’s partner. But then … natural selection would have upped the ante, and favored those who could see through the ruse. Once lying had been invented, there was no turning back. Development would have snowballed.”

I said, “So the fully autistic can’t lie – or judge someone else to be lying. But the partially autistic…?”

“Some can, some can’t. It depends on the specific damage. We’re not all identical.”

“Okay. But what about relationships?”

Rourke averted his gaze, as if the subject was unbearably painful – but he continued without hesitation, sounding like a fluent public speaker delivering a familiar lecture. “Modeling other people successfully can aid cooperation, as well as deception. Empathy can act to improve social cohesion at every level. But as early humans evolved a greater degree of monogamy – at least, compared to their immediate ancestors – the whole cluster of mental processes involved in pair-bonding would have become entangled. Empathy for your breeding partner attained a special status: their life could be, in some circumstances, as crucial to the passing on of your genes as your own.

“Of course, most animals will instinctively protect their young, or their mates, at a cost to themselves; altruism is an ancient behavioral strategy. But how could instinctive altruism be made compatible with human self-awareness? Once there was a burgeoning ego, a growing sense of self in the foreground of every action, how was it prevented from overshadowing everything else?

“The answer is, evolution invented intimacy . Intimacy makes it possible to attach some, or all, of the compelling qualities associated with the ego – the model of the self – to models of other people. And not just possible – pleasurable. A pleasure reinforced by sex, but not restricted to the act, like orgasm. And not even restricted to sexual partners, in humans. Intimacy is just the belief – rewarded by the brain – that you know the people you love in almost the same fashion as you know yourself.”

The word “love” had come as a shock, in the middle of all that sociobiology. But he’d used it without a hint of irony or self-consciousness – as if he’d seamlessly merged the vocabularies of emotion and evolution into a single language.