The Goldsmith Theory of personality types was first published in the 1950s. That study divided personality into a binary of high-strung A-types and easy-going B-types. The study's initial purpose was to find a correlation between personality type (behavior and attitude) and coronary disease. The type D personality was defined by Johan Denollet in the late 1990s and attempts to further refine Goldsmith's theory. He finds that over 20% of the population has what he calls a Type D personality, characterized by social inhibition, worry and anxiety. Around 50% of Denollet's cardiac patients fall into this type D category, linking stress with heart disease.