Is there a correlation between total neurons and intelligence?

Question

Thanks for looking.

First off, I am not a biologist, just a curious layman, so I apologize in advance if this isn't a "good" question. Please don't downvote me into oblivion.

I read today that the human brain has about 100 billion neurons and that got me wondering: is that number pretty standard for everyone or does someone like say, Einstein, have many more neurons than this daft poster.

Question

Is there a correlation between total neuron count and intelligence, or does intelligence depend more on the way neurons are used--or some other factor like previous experience--rather than their total count?

Can this question be answered?

There are two very valid points-of-view posted as answers below, but it sounds like we simply don't have a solid answer to this question at this point in time, so I hesitate to select a "correct" answer.

Answer 1

There are an estimated 100 billion neurons within the human brain. In general a minor variation in the number of neurons should not effect individuals too much, however when there is a more significant loss, such as brain injury or in some forms of dementia cognitive abilities do decline. So in this sense yes the number of neurons does relate to intelligence. However this variation is not what accounts for general variation of intelligence in the population.

Answer 2

I know of no correlation between number of neurons in cortex and intelligence. This question is fraught with controversy because there has been very little work on it but much speculation. Some have suggested that the connectivity between neurons is what is important rather than the number which is logically possible but remains to be supported by definitive evidence.

Answer 3

Unlike a computer, the speed at which any brain can perform a computation is related to the number of synapses it goes through. This means fewer synapses in series correlate with decreased reaction time.

An example of fewer neurons correlating with a decrease in reaction time is exemplified in sensory neurons. All sensory nerve cell bodies are all located in the dorsal ganglia of the spine. Only projections from single nerve cells here reach our tissues. Furthermore, it's only in ganglia, nuclei, and the cortex that we see additional nerve cells. This is because each nerve cell in series introduces latency.

Latency in our sensory and motor circuits is very important to survival. Our intelligence is limited by a pressure to have a desirable reaction time. Reaction time does correlate intelligence. This is why tests of the sort are timed.

There is another factor in intelligence relating to number of neurons: An increase in grey matter means an increase in white matter. There has been some speculation that humans have nearly reached the limit of our cognitive ability, as our ability to store white matter has been reached.

We seem to be occupying the intelligence Goldilocks zone where reaction time, energy consumption, white matter are all in near optimal balance. It's the balance of these things, the sophistication of the circuits themselves that determine intelligence, not neuron count.

Here's a news link about running out of room for white matter

Here's a study which correlates perception time with reaction time

Answer 4

there is no question there is a rough correlation between neuron count and intelligence capability in a general way from looking at the biological species "spectrum" outside of humans. see wikipedia list of animals by number of neurons. eg in wide differences such as comparing insects vs primates etcetera. there is also a rough correlation of neuron count with body mass and total cell count in animals. however within a single species, small variations in neuron count probably do not have much effect on overall intelligence. also it is known that homo neanderthalensis, the ~30k yr precursor to [homo sapiens] had a larger brain cavity and therefore presumably more neurons but apparently less intellectual capacity than homo sapiens, possibly one factor in its extinction due to competitive evolutionary pressures. even various gorilla species have larger brain mass and conceivably correspondingly larger amounts of neurons (under a rough assumption that neuron density per mass would not vary strongly).

it is also thought that brain size has been affected by evolutionary pressure wrt smaller skull size which has to comfortably fit through the birth canal as a constraint. another aspect to study is the enteric nervous system in the human which has a large number of neurons but does not seem to exhibit a large measurable "intelligence" in the classic sense of guiding the animal behavior, but conceivably uses significant processing to manage gastrointestinal control.

another aspect of the question to consider: programmed cell death in the developing nervous system. large numbers of neurons die during normal natal development in "critical windows" and this seems to show that biologically, total number of neurons is apparently not a key measure or controller of intelligence (otherwise evolutionary pressures would tend to decrease this). the nervous system appears to grow "extra" neurons more than "necessary" for smooth functioning of the human organism.