In Eavesdropping: An Intimate History (public library), biolinguist John L. Locke takes a fascinating look at the ubiquitous yet largely unexamined cultural phenomenon, from medieval voyeurism to Hitchcock’s Rear Window to Twitter and Facebook, by way of chimpanzee behavior and bird calls. He points to two conflicting features of eavesdropping that make it particularly interesting: On the one hand, it feeds on activity that is inherently intimate, with the “sender(s)” of the information unaware of its “receiver”; on the other hand, the information relayed is stolen by the receiver rather than donated by the sender.

But one of Locke’s most intriguing insights traces how the evolution of human civilization, and the rise of urbanization in particular, shaped the norms of — and necessity for — eavesdropping:

Our distant ancestors were secure because they could see each other at all times. They were either trusted, or did not need to be. But on the way to modernity many things happened. A sequence of factors — from sedentism and population growth to the construction of durable housing — nudged our ancestors along a path that could only lead to long periods of personal opacity. The process took many millennia but only began to seriously impact supplies of social knowledge in the last several thousand years.

When residential walls were erected, it was the beginning of truer and deeper forms of intimacy. Walls also made it difficult — and ultimately unnecessary — to look around every few seconds to see what others were doing. A human vigil, one beginning with ancestors that we share with apes, was reduced to manageable proportions, freeing up many hours of undistracted time per day. This would gradually increase opportunities to develop the kind of personal, marital, and familial relationships that we now hold dear.

At one time, the isolation-cum-privacy enabled by walls was about as welcome as incipient blindness. By blocking the eye, walls placed a premium on something that they knew very little about: trust. What was trust? Who could be trusted? With so few previous opportunities to violate trust, it was hard to tell. Predictably, suspiciousness and fear rose precipitously. If walls were to continue, more penetrant means of perception would be needed. Fortunately, a suitable cognitive mechanism was waiting in the wings.

It was eavesdropping, a term that I will use in its conventional sense to mean surreptitious observation as a technique for sampling the intimate experiences of others — whether the surveillant is peeking through a keyhole or just feigning inattention to ambient activity. But I also use the term metaphorically to represent the lifelong quest of all humans to know what is going on in the personal and private lives of others.