The most widely known academic model of needs was proposed by psychologist, Abraham Maslow, in 1943. His theory proposed that people have a hierarchy of psychological needs, which range from basic physiological or lower order needs such as food, water and safety (e.g. shelter) through to the higher order needs such as self-actualization. People tend to spend most of their resources (time, energy and finances) attempting to satisfy these basic before the higher order needs of belonging, esteem and self-actualization become meaningful. Maslow's approach is a generalised model for understanding human motivations in a wide variety of contexts, but must be adapted for specific contexts. While intuitively appealing, Maslow's model has been difficult to operationalize experimentally. It was developed further by Clayton Alderfer.