[tl;dr — we really don’t know much about most mental illness or how to make drugs that help it. -egg]
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Having been discovered by accident, however, [psychiatric drugs] lacked one important element: a theory that accounted for why they worked (or, in many cases, did not).
That didn’t stop drug makers and doctors from claiming that they knew. Drawing on another mostly serendipitous discovery of the fifties—that the brain did not conduct its business by sending sparks from neuron to neuron, as scientists previously thought, but rather by sending chemical messengers across synapses—they fashioned an explanation: mental illness was the result of imbalances among these neurotransmitters, which the drugs treated in the same way that insulin treats diabetes.