Brain size: is bigger better? or, Of mice and men (and elephants)

What would you say if I told you there may be a way to increase your brain size?  Would you be interested?

If you are, you're probably not alone, but would having a bigger brain make you smarter? Or would you just be throwing your money away?

Sadly, most of the evidence suggests that overall brain size is not a critical determinant of intelligence or cognitive function, and in some cases, having a bigger brain can actually be a bad thing.

Now, I was planning on writing a nice long post about how bigger is not always better when it comes to the brain, particularly when we look at different animals, but, I just read this article on ScienceDaily (http://www.sciencedaily.com/releases/2009/11/091117124009.htm) that sums things up pretty well.  I'll just give a quick example that I think is quite convincing:
the brain of an Asian elephant is obviously larger than that of your average human (about 7.5 kilograms, where the human brain is, on average, a little less than 1.5 kilograms).  Though, I think we will all agree that humans are smarter than elephants (if only slightly).  To get around this blatant difference between humans and elephants, some people have argued that proportional brain size is what matters. That is, if you take the size (or mass) of the brain and divide it into the size of the whole animal, you get a ratio that describes the size of the brain in proportion to the the rest of the body.  This works well when you look at humans and elephants, where, in humans, the mass of the brain over that of the body gives a ratio of about 2.1%, and in Asian elephants, about 0.15%. And to some extent, this approach works for other comparisons, where there are several examples that make sense (the ratio is larger in "smarter" animals),  HOWEVER... there are definitely many examples where this doesn't work, for instance, when you look at mice, the ratio of brain weight over bodyweight is 3.2%, about one and a half times more than the ratio in humans.  So, if bigger is always better, then either mice or elephants are smarter than people... which, despite the popularity of Sarah Palin's new book, is still highly doubtful.

So, comparisons across different species suggest that bigger brains don't equal smarter animals (even when you take overall body size into account).  But what about if you compare humans to other humans?  Here, there is some evidence that bigger is better (that's what she said), though again there are many exceptions and reasons for questioning that evidence (he retorted defensively).  For example, when we look at the fossil record, we see that, as humans evolved, our skull cavities got bigger, suggesting that our brains got bigger as we got smarter (though, again there are exceptions, like Neanderthals whose brains may have been bigger than our own).  When we look at more recent evidence in humans, we see some interesting things.  For example, boys have larger brains (on average) than girls (which remains the case in adulthood as well).  But does that mean boys are smarter than girls?  Well, if we look at average SAT scores, boys do tend to score higher than girls, BUT, if we look at grades (in co-ed institutions) girls get better grades than boys do.  So where does that leave us?  It leaves us with no clear evidence that one sex is any smarter than the other, despite the difference in brain size.  (The funny thing is, when we look at brain size as a proportion of total body size, girls actually have bigger brains than boys do, of course that still doesn't help us).
If we try to ignore the confusion of comparing men and women (or boys and girls as the case may be), there are some reports (based on data from MRI scans) that suggest bigger brains do correlate with higher IQ scores (which are certainly a limited, and perhaps biased measure for intelligence, but still interesting, and not completely irrelevant).  However, these are weak correlations, and they do NOT demonstrate a hard and fast rule (see the very end of this post for a more detailed explanation of what I mean).  For example, though he wasn't included in the MRI studies, one famous exception to this idea is Albert Einstein.  When pathologists examined Einstein's brain after his death they found that it was not any larger than your average human brain, though I'd be willing to bet he would score pretty high on an IQ test. 
In addition to the fact that these correlations were weak, there are a couple other points that make this evidence questionable (as to what it really means, if anything)...
1. several variables can change brain size over short periods of time.  For example, drinking alcohol temporarily shrinks the brain, which means if anyone in these studies was hungover, they would likely appear to have a smaller brain and also probably wouldn't do so well on the IQ test.  Other factors that are known to affect brain size include diet, exercise, marijuana, medication, and even meditation.  Given the relative ease with which these things can alter brain size, it seems more difficult to tie any measure to absolute brain size since brain size is obviously not absolute.
2. IQ tests may not indicate a person's overall intelligence.  That is, someone with a smaller brain may not have done as well on an IQ test, but they may be much more skilled in some other form of intelligence, like art (painting, sculpture, or music, etc.).  We all know that Mozart was a genius (whose music can actually make you perform better on IQ type tests) and yet, he would probably not score so high on an IQ test.
3. Correlation is not cause.  I have talked about this before.  Even if we assume the data is unequivocal (that is, an almost perfect correlation, where 100 percent of the time a bigger brain is associated with greater intelligence) we still can't say that having a bigger brain causes people to be "smarter".  Perhaps, brains get bigger with use, and therefore, people who study more or who have more years of education (and can therefore score higher on an IQ test) have slightly larger brains. For example, a study conducted about a decade ago with London cab drivers suggested that, at least a part of the brain might be able to get bigger with use.  The study showed that the hippocampus (which is important for remembering where things are located) was bigger in cab drivers than in regular commuters, and cabbies who had been on the job for many years had larger hippocampi than cabbies who had only been working for a shorter time.  While this isn't definitive, and doesn't say anything about other parts of the brain, it does suggest that brain growth in response to use or practice is a reasonable hypothesis.


So, what's the answer?  Are bigger brains better? I think, despite the correlations between brain size and IQ, we still have to say NO.
One final piece of evidence I would like to offer is the presence of a condition (in humans) called megalencephaly, which literally means "large brain".  Megalencephaly is characterized by a brain that is unusually large or heavy when compared to average brain size, and while it is suspected that the causes are genetic, they are not fully known.  Interestingly, megalencephaly often results in decreased intelligence and mental retardation, which seems pretty convincing evidence that simply having a bigger brain does not confer any improvements in cognitive abilities. Of course, another condition known as microencephaly, or "small brain", also results in mental retardation and, often, early lethality.  Together, these extreme conditions suggest that our brains work best within a range of sizes that are reasonably close to average.  So if you have an average sized brain, be grateful, and know that, you're in good company (with Einstein), and while there may be other factors determining how smart you are (environmental factors like education level, or diet and exercise, or genetic and biological factors, like how many receptors for neurotransmitters your nerve cells make), brain size is likely not one of them.

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The statistical measure for a correlation is the r value.  In the MRI studies the r values ranged from 0.35 to 0.51, which may be statistically significant, but suggest a fair amount of deviation (what we would call a "weak" correlation).  "r" values range from -1 to +1, with -1 being a perfect negative correlation (one thing gets smaller while the other gets bigger), +1 being a perfect positive correlation (two values get bigger together), and 0 being no different from random chance (the two variables don't seem to be related to each other at all). While getting an r value of 0.4 or 0.5 is definitely meaningful (what we call "statistically significant"), it suggests that there are a fair number of people who break from the trend. "Significance" in terms of statistics, signifies that the relationship being studied (in this case between brain size and IQ score) is not random, or likely doesn't reflect measures you would get by chance.  To give you a simple example of what it means to be non-random, or not by chance, think of the simple comparison between two individuals where chance for a given variable is 50 percent (like flipping a coin, 50 percent chance of heads, 50 percent chance of tails).  If you come across two people with different brain sizes, the MRI data suggests it is likely that more than fifty percent of the time, the person with the bigger brain will score better on an IQ test than the person with the smaller brain.  However, a greater than 50 percent chance is NOT a hundred percent certainty.  So, some of the time (less than 50 percent, but still greater than 0 percent), the person with the smaller brain will be "smarter" than the person with the larger brain (at least when measured by IQ).