Monday, November 8, 2010


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(Photo: Leslie Reeder)

Smithsonian Institution


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People love to speculate about differences between the sexes, and neuroscience has brought a new technology to this pastime. Brain imaging studies are published at a great rate, and some report sex differences in brain structure or patterns of neural activity. But we should be skeptical about reports of brain differences between the sexes, writes psychological scientist Cordelia Fine in Current Directions in Psychological Science, a journal of the Association for Psychological Science. The results from these studies may not necessarily withstand the tests of larger sample sizes or improved analysis techniques—and it’s too soon to know for sure what such results, even if they prove to be reliable, might mean for differences in male and female minds.

Bookstores are full of popular books on the differences between men’s and women’s brains. Fine, who works at Macquarie University in Australia, first became interested in the issue as a parent. She was reading a book about how the differences between boys’ and girls’ brains mean they should be taught differently. But as an academic, she was curious about the research on which these claims were based, and looked up the original studies.

“There were huge discrepancies between what the neuroimaging studies showed and the conclusions and claims that were being drawn from them,” she says. In the article and her new book, Delusions of Gender, Fine dissects the ways that research goes astray between the scanning machine and the sound bite.

Some of the problems start with the research. The studies Fine came across were often conducted with small numbers of men and women, where the differences seen could have been due to chance. It’s very easy and obvious for neuroscientists to compare the sexes by default. But when neuroscientists habitually check for sex differences, some researchers, just by chance, will find statistically significant differences between the two groups—even if there’s no real difference between men and women overall.

This problem of false positive results is understood by the neuroscientists who do the research; they know that one study with 20-odd participants that finds some small region of difference between males and females is not the final word on the issue. But these often subtle, questionable differences are readily seized on by popular writers, Fine says.

Another problem is how to interpret sex differences in the brain. Neuroscientists are only beginning to understand how neural activity brings about complex psychological phenomena. The temptation, to which popular writers are particularly vulnerable, is to use gender stereotypes to bridge that gap in scientific knowledge.

The fact that neuroimaging studies use complex, expensive machines that seem to take pictures of the brain may also make their results seem more real, reliable, and impressive than behavioral studies. As a result, substantial behavioral evidence of gender similarity, or the sensitivity of gender differences to context, can be overshadowed by a single finding of a sex difference in the brain.

“A healthy dose of skepticism is required when it comes to reports of sex differences in the brain and what they mean,” says Fine, who is concerned that claims about differences in male and female brains are reinforcing old-fashioned gender stereotypes.

Association for Psychological Science


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The findings have implications for modern-day global warming, said Ellen Martin, a UF geological sciences professor and an author of the paper, which is published in the journal Nature Geoscience.

“It helps us understand how the carbon cycle works, which is important for understanding future global warming scenarios,” she said. “Ultimately, a lot of the carbon dioxide that we’re pumping into the atmosphere is going to end up in the ocean. By understanding where that carbon was stored in the past and the pathways it took, we develop a better understanding of how much atmospheric carbon dioxide the oceans can absorb in the future.”

Scientists know that during the transition from the last glacial period to the current inter-glacial period about 14,000 years ago, carbon dioxide levels rose very quickly at the same time that the age of the carbon dioxide in the atmosphere fell, as measured by radiocarbon data. That suggests carbon dioxide had been stored in the ocean and suddenly released, she said.

One idea holds that it was building up in the Southern Ocean around Antarctica, where extensive sea ice on the surface of the ocean initially prevented the exchange of gasses into the atmosphere, Martin said. The other possibility is that the same process occurred in the Northern Hemisphere with ice sheets in the North Pacific Ocean, she said.

In her lab, Martin and lead author Chandranath Basak, a UF graduate student in geological sciences; Keiji Horikawa, a UF postdoctoral fellow in geological sciences; and Thomas Marchitto, a University of Colorado geology professor, studied that question by using a technique to measure isotopes of neodymium, a rare earth element not commonly found in marine sediments but preserved in microscopic fossil fish teeth. The isotopic signature of a water mass, which is captured in the fish teeth, reflects the location where the water mass came from, she said.

“It’s essentially what we call a water mass tracer,” Martin said. “You can tell where the water masses have formed and where they have moved to by using this tracer.”

The researchers took samples that had been shown to have old carbon in them and measured the neodymium isotopes on fish teeth from the sediments to see if they could reconstruct whether they had come from the North Pacific or the Southern Ocean, she said.
“When we did this, we got a signal that looks very much like the Southern Ocean,” she said. “It implies that all the carbon was being stored in the Southern Hemisphere and as the ice sheet melted back, it released that carbon dioxide into the atmosphere, causing part of the big increase in carbon dioxide and introducing old carbon back into the atmosphere.”

By giving information about environmental conditions during the last glacial period, the research findings can help scientists to reconstruct what the world was like at that time, she said.

The implications are that while large amounts of carbon could be stored in the ocean when there was a great deal of sea ice, the opposite is the case in a world that is warming, with less ice, which allows more carbon dioxide to be released into the atmosphere, Martin said. Thus, in a warming scenario the oceans may not be able to store as much carbon dioxide as they could under glacial conditions

The oceans are a critical part of the carbon dioxide cycle, Martin said. “The oceans have 60 times more carbon dioxide in them than the atmosphere, so when we worry about what’s happening with carbon dioxide in the atmosphere, we often look to the oceans as a potential source or sink.”

The concentration of carbon dioxide in the atmosphere during the glacial periods was about 200 parts per million, compared with 280 parts per million during a typical interglacial period, Martin said. Today that level has soared to about 380 parts per million, she said.

The time period that encompasses the last glacial period to the current interglacial period when carbon dioxide levels went up very quickly is often referred to as the “mystery interval” because scientists hadn’t known where the carbon was stored, Martin said.

“Now we have a better understanding of how the system worked,” she said.

University of Florida



