New research suggests that the layer of insulation coating neural wiring in the brain plays a critical role in determining intelligence. In addition, the quality of this insulation appears to be largely genetically determined, providing further support for the idea that IQ is partly inherited.

The findings, which result from a detailed study of twins’ brains, hint at how ever-improving brain-imaging technology could shed light on some of our most basic characteristics.

“The study answers some very fundamental questions about how the brain expresses intelligence,” says Philip Shaw, a child psychiatrist at the National Institute of Mental Health, in Bethesda, MD, who was not involved in the research.

The neural wires that transmit electrical messages from cell to cell in the brain are coated with a fatty layer called myelin. Much like the insulation on an electrical wire, myelin stops current from leaking out of the wire and boosts the speed with which messages travel through the brain–the higher quality the myelin, the faster the messages travel. These myelin-coated tracts make up the brain’s white matter, while the bodies of neural cells are called grey matter.

White matter is invisible on most brain scans, but a recently developed variation of magnetic resonance imaging, called diffusion-tensor imaging (DTI), allows scientists to map the complex neural wiring in our brains by measuring the diffusion of water molecules through tissue. Thanks to the fatty myelin coating, water diffuses along the length of neural wires, while in other types of brain tissue it moves in all different directions. Researchers can calculate the direction of fastest diffusion at each point in the brain and then construct a picture of the brain’s fiber tracts. A well-organized brain has well-functioning myelin, in which water can be seen clearly moving along specific paths. “Diffusion imaging gives a picture of how intact your brain connections are,” says Paul Thompson, a neuroscientist at the University of California, Los Angeles, who lead the study.

Thompson and his colleagues took DTI scans of 92 pairs of fraternal and identical twins. They found a strong correlation between the integrity of the white matter and performance on a standard IQ test. “Going forward, we are certainly going to think of white matter structure as an important contributor of intelligence,” says Van Wedeen, a neuroscientist at Massachusetts General Hospital in Boston, who was also not involved in the research. “It also changes how you think about what IQ is measuring,” says Wedeen. The research was published last month in the Journal of Neuroscience.