People - like plants - can only blossom when the conditions are right.
What do people need? Well, it depends on the circumstances. If you're living in poverty in a famine area, your most important need is food. On the other hand, if you're living in physical comfort but are isolated from people you know, your top priority will be gaining access to friends or family.
So people have needs depending on their circumstances. This is the basis for Maslow's Hierarchy of Needs, one of the best-known theories of motivation. It argues that there are five levels of need, and that these are hierarchical, such that lower level needs must be satisfied before higher ones. Maslow's five levels of need (starting with the lowest) are: physiological, safety, social, self-esteem and self-actualization.
While it's a useful starting point, Maslow's theory doesn't fully reflect the true complexity of human motivation. Using the Hierarchy of Needs, our physiological need for food would have to be met before we felt the need for social relationships. In reality, these needs are usually not as independent as that: You can be hungry for love and food at the same time. Likewise, you can experience a need to belong (social) at the same time you are looking for challenging work (esteem).
The psychologist Clayton Alderfer developed a new model to explain the simultaneous nature of Maslow's five needs. Called the ERG Theory of motivation, he first published it in a 1969 article titled "An Empirical Test of a New Theory of Human Need."
In his theory, Alderfer compressed Maslow's hierarchy of needs from five to three:
(Hence "ERG", standing for Existence, Relatedness and Growth.)
At the most basic level, people have existence needs. These map to Maslow's physiological and safety needs, as shown in Figure 1 below.
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