Memo to tennis players: because of the way the human visual system works, referees are more likely to call “out” a ball that actually lands in, rather than call “in” a ball that in fact lands outside the line. Now that professional players are permitted to challenge calls, therefore, they would do well to focus on balls that are called “out,” since they are more likely to be wrong.

So concludes a neat little study published online today in Current Biology. Scientists led by David Whitney of the University of California, Davis, started from the fact that the human visual system consistently misperceives moving objects as shifted in the direction of their motion, making them appear to be farther along their path than they are.

To see what effect this might have in a tennis match, Whitney and his colleagues analyzed 4,457 randomly-selected points from the 2007 Wimbledon tournament, focusing on those where the ball landed close to the line. Using video from the matches, they uncovered 83 incorrect calls. Of those, 70 were balls that were called out when they actually fell inside the line. Only 13 were called in when they were actually out. Prediction confirmed: the brain thinks a moving object is farther along in its path that it really is.

Under the new rules for challenges, players get two (in the U.S. and Australian Opens) or three (at Wimbledon) incorrect challenges per set. Given the bias in incorrect calls, it is clearly to a player’s advantage to focus on balls that are called “out” in their opponents’ court rather than balls called “in” on their own side of the net: the former challenges are more likely to be upheld, while the latter count against the two or three incorrect challenges a player is allowed. As long as a player continues to challenge incorrect referee calls, he or she is allowed to continue making challenges. (At Wimbledon in 2007, there were about 140 challenges, resulting in 25% of the calls being overturned.) Players can improve their odds of challenging only incorrect calls if they focus on balls called “out.”

In our gene-obsessed society, whenever one group differs from another on some measure of health, laypeople as well as experts reflexively leap to a genetic explanation. Higher rates of hypertension among African-Americans than Caucasian Americans? It must be their genes. That assumption is behind race-based medicine, too—the idea that different medications will work better for people of different races.

Next time someone hands you that line, send them to a paper published in the November issue of the journal Social Science and Medicine.

In it, scientists led by Roland James Thorpe of the Bloomberg School’s Hopkins Center for Health Disparities Solutions, while not ruling out genetic or other inherent biological factors, find that social environment is a big reason for the black-white hypertension disparity. “Our study found that nearly 31 percent of the hypertension disparity among African Americans and non-Hispanic whites is attributable to environmental factors,” Thorpe said. “These findings show that ethnic disparities could be linked to a number of factors other than race. Careful review of psychosocial factors, stress, coping strategies, discrimination and other personality characteristics could play a large role in reducing or eliminating the disparity.”

Translation: Not to put too fine a point on it, but living in a society where security guards at high-end stores eye you suspiciously, where cab drivers refuse to pick you up, where fellow students and work colleagues let you know in subtle and not-so-subtle ways that they think you’re where you are because of affirmative action—well, let’s just say that any and all of these are excellent ways to raise your blood pressure. Hypertension, in turn, can damage the heart and blood vessels, raising your risk of stroke, heart failure, heart attack and kidney failure. It affects some 65 million people in the U.S.

In a nutshell, the Hopkins scientists compared data from a study called the Exploring Health Disparities in Integrated Communities-SWB (EHDIC-SWB) Study, which was run in a racially-integrated community where income did not vary by race, with data from a broad-based national survey called the National Health and Nutrition Examination Survey. They focused on hypertension, defined as systolic blood pressure of 140 or more, or diastolic blood pressure of 90 or more. Comparing the two data sets, the scientists found that race alone elevated blood pressure about one-third less in the racially-integrated community than nationally.

“These findings support our theory that the disparity [in rates of hypertension] is likely caused by environmental factors along with several external factors, and not biological differences among race groups, as previously suspected,” said co-author Thomas LaVeist.

It's great to live in an integrated (and presumably less racist than the national average) community as an adult, but biological changes triggered by social experiences kick in long before adulthood. So, here’s the next question: what would happen to race-based health disparities if children grew up in a race-blind society, never feeling the blood-pressure-elevating sting of racism? Just wondering.