Emotional life can be influenced, but it cannot be commanded. - Thomas Lewis

On the other side of the aisle are evolutionary psychologists and cross-cultural sociologists, who point out that the American habit of sleeping separately is a global and historical singularity. Almost all the world’s parents sleep with their children, and until the last sliver of human history, separate sleep was surpassingly rare. The burden of proof thus falls upon our culture to justify its anomalous nighttime practices.

When people have trouble with their emotions – a bout of anxiety or depression, say, or seasonal gloominess - they often want science to pinpoint an offending neurotransmitter in the way that a witness picks the perp out of a lineup. Is it excessive norepinephrine, too little dopamine, errant estrogen? The answer is apt to dissatisfy: no single suspect can be fingered with confidence because the question itself attributes a fallacious simplicity to the brain.

The neurally ingrained Attractors of one lover warp the emotional virtuality of the other, shifting emotional perceptions — what he feels, sees, knows. When somebody loses his partner and says a part of him is gone, he is more right than he thinks. A portion of his neural activity depends on the presence of that other living brain. Without it, the electric interplay that makes up him has changed. Lovers hold keys to each other’s identities, and they write neurostructural alterations into each other’s networks. Their limbic tie allows each to influence who the other is and becomes.