Two huge medical studies wrap up. Both assess the value of screening for prostate cancer. Both are published in the same edition of the New England Journal of Medicine. One of the studies is American. It finds there is no difference in the mortality rate of men screened for prostate cancer and those who are not. The other study is European. It finds screened men are less likely to die, although the difference is modest. Prostate cancer is a hot topic and these are landmark studies. So how will the results be reported? If you think the studies were given equal attention in the media, with headlines as mixed as the science, you are a very logical person. You are also very wrong. In the United States and Australia, news coverage of the two studies -- published March 19 -- gave the American study more prominence. But in the United Kingdom, the European study was prominently reported, while the American study was ignored entirely. (The studies got little notice in Canada.) Is this geographic chauvinism? Not unless Australia has drifted across the Pacific. Besides, the two studies were published in the same (world-famous) journal. I think there's something else going on here. And it has profound implications, not only for what does and doesn't make the news, but for our very perceptions of reality. Here's a simple but crucial fact: The prostate screening which was the subject of the two studies is already in widespread use in the United States and Australia, but not in the United Kingdom. By reporting only the European study (the one that found screening saves lives) the U.K. media made this a "bad news" story: screening saves lives; the U.K. doesn't screen; therefore British men are dying needlessly. "Better Cancer Screening Is Every Man's Right" bellowed the headline of an editorial in the Scotsman. But in the U.S. and Australia, emphasizing the American study (that said screening does not save lives) made it a "bad news" story: We're spending all this money and putting men at risk of complications for something that doesn't even help. "PSA Testing May Not Save Your Life After All," as the headline in Scientific American put it. Heads, it's bad news. Tails, it's bad news. See the trend? This isn't the first time back-to-back medical studies have revealed the media's "bad news" bias. In 1991, an issue of the Journal of the American Medical Association carried two studies about childhood cancers caused by radiation. One study said the radiation did cause cancer. The other said it did not. Researchers found 19 news stories reported the studies. None mentioned only the good-news study. Nine reported only the bad-news study. The remaining 10 reported both -- but all 10 gave more space and prominence to the bad news than the good. Of course, it's not news to anyone that the media prefer bad news. But I don't think that people appreciate how profound the bias is. Nor do they grasp its ultimate cause. Typically, the media are accused of consciously twisting the news in order to sell more newspapers. In a brilliant and savage dissection of the -- truly awful -- reporting of the prostate studies in the U.K., British physician and skeptic Ben Goldacre accused reporters of "deliberately" ignoring one of the studies and exaggerating the other. Maybe they did. I don't know. But, having worked in a newsroom, I find it very hard to believe that every one of the many reporters and editors handling this story consciously chose to do something so unprofessional and unethical. I find it particularly hard to believe because the reporting was also biased in the U.S. and Australia. Did all those American and Australian journalists calculatedly choose sensationalism over accuracy? I doubt it. I especially find it hard to swallow because there is a simpler and more powerful explanation. Psychologists call it "negativity bias." Bad trumps good. It's that simple. "As a general principle across a broad range of psychological phenomena," wrote four psychologists in a review of the research literature, "bad is stronger than good." In one study, the boost in happiness people experienced after a good event was found to dissipate much more quickly than the loss of happiness experienced following a bad event. Other research found good days had no effect on mood the following day, while bad days had lingering effects. People reacted more strongly to bad odours than good. Losses were felt more keenly than gains. And bad interactions in a marriage were shown to have more effect on relationships than good -- by one researcher's estimate, it takes five positive interactions to balance one bad one. A review of psychology textbooks found twice as many chapters devoted to bad emotions as good. A survey of research papers dealing with emotions found 69 per cent dealt with bad emotions and 31 per cent with good. Even our language is biased: Researchers who compiled a list of every English word with an emotional connotation found 62 per cent involved bad feelings versus 38 per cent for good. Show people pictures of smiling and frowning faces and their eyes will be go first to the sourpuss. Bad words ("sadistic") grab attention better than good words ("honest") -- and are more likely to be remembered. These studies further revealed a crucial point: People are not aware that their attention and memory are biased toward the negative. "The greater impact of bad than good is extremely pervasive," the 2001 review concluded. Why would this be? When a phenomenon is as universal and sweeping as this is, it suggests hardwiring. And that means evolution. "It is evolutionarily adaptive for bad to be stronger than good," the reviewers believe. "Survival requires urgent attention to possible bad outcomes, but it is less urgent with regard to good ones." If your ancestor was picking berries on the savannah one afternoon 100,000 years ago, he could carry on if someone mentioned that a lion hadn't been spotted for days. But when someone said "I think I see a lion," he had to drop the berries and pay close attention if he wanted to survive, reproduce, and become your ancestor. Bad news mattered more. So, yes, the media are profoundly biased toward bad news. And, yes, sometimes that tilt may be the result of crass attempts to sell more newspapers and draw bigger audiences. But fundamentally, journalists are drawn toward bad news for the same reason readers and viewers are: They are human.