Carl Gustav Jung, the Swiss-born psychiatrist, developed the modern concept of psychological types, preferences with which individuals are born that form the foundation of their personalities. Soon after Jung's work appeared in English translation, an American researcher, Katharine Briggs, began detailed studies of Jung's work. She, along with her daughter Isabel Briggs Myers, devoted nearly two decades to developing ways to measure the preferences of individuals in order to determine their types and the strength of their preferences. Their collaboration resulted in the creation of a survey instrument, the Myers-Briggs Type Indicator (MBTI), which has been given to millions of Americans under the guidance of professionals trained in the administration and interpretation of the instrument. David Keirsey later developed a similar but less detailed lay instrument called the Keirsey Temperament Sorter which appeared in his 1984 book, Please Understand Me. That work had sold over one million copies by 1991, and the Sorter has been administered to additional millions.