IRA FLATOW, HOST:
This is SCIENCE FRIDAY, I'm Ira Flatow. Turn on reality TV, and it may not be long before you ask yourself: Are we getting dumber? A new study may have some genetic answers to that question. Provocative research published this week in the journal Trends in Genetics suggests that human intelligence may have peaked thousands of years ago.
Gerald Crabtree from Stanford University studied mutations in our genes and found that human intelligence may have started slowly declining ever since we moved away from the hunter-gatherer ways of our ancestors. So is our modern lifestyle to blame for this brain drain? And if we're getting dumber, why is the average IQ on the rise?
Give us a call, our number 1-800-989-8255. You can also tweet us @scifri. Gerald Crabtree is the David Korn Professor of Pathology and Developmental Biology at Stanford School of Medicine. He's also an investigator at the Howard Hughes Medical Institute. Welcome to SCIENCE FRIDAY.
GERALD CRABTREE: Good morning, Ira.
FLATOW: Good morning to you. What evidence do you cite? What evidence is there that human intelligence is slowly declining?
CRABTREE: Well, you know, there are a few things I'd like to say I guess at the onset, and you referred to them in your introduction, and that is any genetically based decay in intelligence is extremely slow. And so we should never be able to detect it by comparisons to people within generations existing right now on the Earth.
Rather, what we see is what you mentioned, an actual, probably recent, increase in intellectual ability. We know that our students are the brightest in generations. They score higher on tests than students ever have before. They take more complex courses earlier and accomplish more. So there's all these reasons to believe that at least recently what we see is an actual increase in intellectual ability.
And these are due to major - or thought to be due to major societal changes that need to be applauded and mentioned. They are things like better nutrition, better maternal health care, better monitoring of fetal health during deliveries, the elimination of lead from gasoline and paint, the reduction in mercury from the effluent of power plants.
All these things have contributed, probably, to a recent increase in intelligence, even societal things such as the way that students are taught allow them to score better on tests. So that's the predominant influence that we should see in looking around us at our contemporaries and not this slow process that I've been - that I've written about, which it takes really generations, hundreds of years, even millennia, to really make an important difference.
FLATOW: So you can tell this from looking at gene mutations over those millennia?
CRABTREE: The reason that I wrote this was really because in looking at other people's data - this is not my own data; I just more or less synthesized what was out there - there have been a few developments that allow one to do things that have always been speculated upon before but now are known quite precisely.
So one of them is the germ-line human mutational rate, and what that is is the frequency with which new mutations occur in our children. And so the technique that has allowed this to be assessed accurately is one called trio sequencing, where the mother, the father and the children each have their DNA sequenced, their genetic material.
So all three of the nucleotides are sequenced, and what that allows one to do is to identify any new changes that have occurred. And so that tells us what the number of random mutations are that occur in the genome with each new generation. These new mutations mark a generation for eternity.
FLATOW: I'm trying to get to the point here. How does that tell you that we're getting dumber over all these years?
CRABTREE: Well, it's a long argument, and the problem with this argument is that it's largely a mathematical argument. So this is one footing of the argument that is the recent determination of precise rates of mutation, so it hadn't been possible before, it had been estimated, but now we know precisely. So that gives us a rate at which random mutations occur in the genome.
The next fact that - or it's not really a fact, but it's close to it, is the number of genes required for our normal cognitive abilities, our normal intellectual abilities. And this has come about through a variety of means. Recent studies using full genome sequencing and other things, and other approaches, have allowed one to estimate the number of genes at risk for these new mutations.
So for example one can estimate the number of genes that are required for normal human intellectual ability by looking at the X chromosome.
FLATOW: And what do we find from examining all of these?
CRABTREE: So if we look at the X chromosome, what we see is that there are a certain number of mutations that occur there. The X chromosome is an interesting one because of course in males we have just X and Y, where females have X and X, so they're protected from any changes, females are protected from any changes in the mutations in the X chromosome.
So by looking at the number of what's called X-link cognitive disorders, one can estimate, based upon the number of genes on the X chromosome, the total number of genes in the human genome that would give rise, if mutated, to some form of cognitive deficiency.
And so from that you get a number of genes of about somewhere between about 2,000 and 5,000. So that becomes now the target upon which these, let's say, 40 new mutations per genome per generation are operating.
FLATOW: So we're getting 40 new mutations per generation in these 2,000 necessary genes for intelligence is what you're saying, and over the millennia, these things add up.
CRABTREE: Exactly. I mean, that is - you made one jump there that is not correct, and that is that the new mutations are not necessarily in the intelligence genes. They're just spread out randomly over the genome.
FLATOW: I see.
CRABTREE: They have a certain probability of hitting these - this group of genes that are the cognitive genes so that among those genes, we know that they behave like links on a chain, that is if any one of them is mutated there is a cognitive problem.
And so that means then...
FLATOW: Oh, I think we lost him. We lost Dr. Crabtree. Well, OK, live radio, that's what happens sometimes. Transcript provided by NPR, Copyright NPR.