The Baskerville effect, or the Hound of the Baskervilles effect is a statistical observation that mortality through heart attacks is increased by psychological stress. The term derives from the Sherlock Holmes novel The Hound of the Baskervilles in which a fierce big black dog is souped up to make it look like the dog breathes fire in the night; the hellish-looking dog chases a man, who runs in fear, and the man breaks his neck.
The Baskerville effect was discovered by David Phillips and his colleagues at the University of California, San Diego, who found that daily number of deaths of the 200,000 Chinese, Japanese and Americans who died from heart attacks between 1973 and 1998 was 7% higher on the fourth of the month compared to the average for the other days in that month.
Four (四, formal writing: 肆, pinyin si4) is considered an unlucky number in Chinese, and hence in the Japanese and Korean, because it sounds like "death" (死 pinyin si3). Some Chinese and Japanese hotels and hospitals do not use it as a room number.
His hypothesis was that the peak was caused by stress induced by the superstition surrounding this number. Previous research had also shown a complementary effect, mortality falling before auspicious occasions and rising again afterwards.