Questioning Big Round Numbers: The Brain has How Many Neurons?

Most of us have read over and again that the human brain has 100 billion neurons.  With no source but legend for that big round number—and not wanting merely to echo an undocumented estimate from other books—I set off in search of a more precise estimate.  Surely someone must have sampled brain tissue, counted neurons, and extrapolated a nerve cell estimate for the whole brain.  (It’s not that the number affects our understanding of how the brain works, but we might as well get the facts right.)

One researcher whose name I was disposed to trust—Gabrielle De Courten-Myers—explained to me by e-mail how she used “histological neuronal density and cortical thickness measurements in 30 cortical samples each from 6 males 12 to 24 years old,” from which she extrapolated an estimate of 23 billion neurons for the male cerebral cortex.  Although she didn’t have data for the rest of the brain, her guess in 2005 was that a whole-brain total would be “somewhere around 40 billion neurons.”

Later, a different research team, using a method that is beyond my pay grade to understand (but apparently involved making a “brain soup” of four male brains, postmortem, and counting neural nuclei) estimated 86 billion neurons in the male brain (though yet another expert with whom I corresponded questioned the validity of their method).

So, how many neurons have we in our human brains?  Apparently something less than 100 billion, but the number is uncertain.  What’s more certain is that we should be suspicious of unsourced big round numbers:  “The brain has 100 billion neurons.” “Ten percent of people are gay.”  “We typically use but 10 percent of our brains.”