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Courtesy of http://www.moratmarit.com/2009/08/mcclellands-need-achievement-theory.html
McClelland's Theory of Needs

In his acquired-needs theory, which draws on Murray's model, David McClelland proposed that an individual's specific needs are acquired over time and are shaped by one's early life experiences. Most of these needs can be classed as either achievement, affiliation, or power. A person's motivation and effectiveness in certain job functions are influenced by these three needs. McClelland's theory sometimes is referred to as the three need theory or as the learned needs theory. Later work indicated that motives are actually quite stable over long periods of time.
Achievement
People with a high need for achievement seek to excel and thus tend to avoid both low-risk and high-risk situations. Predominantly Achievement-motivated individuals avoid low-risk situations because the easily attained success is not a genuine achievement. In high-risk projects, the Achievement-motivated see the outcome as one of chance rather than one's own effort. High individuals prefer work that has a moderate probability of success, ideally a 50% chance. Achievement-motivated individuals need regular feedback in order to monitor the progress of their achievements. They prefer either to work alone or with others like themselves.
Affiliation
Those with a high need for affiliation need harmonious relationships with other people and need to feel accepted by other people. They tend to conform to the norms of their work group. High individuals prefer work that provides significant personal interaction. They enjoy being part of groups and when not anxious make excellent team members, though sometimes are distractible into social interaction. They can perform well in customer service and client interaction situations.
Power
A person's need for power can be one of two types - personal and institutional. Those who need personal power want to direct others, and this need often is perceived as undesirable. Persons who need institutional power (also known as social power) want to organize the efforts of others to further the goals of the organization. Managers with a high need for institutional power tend to be more effective than those with a high need for personal power. Work by Abigail Stewart indicated that this motive can interact with emotional maturity; at Stage I, one feels powerful by being associated with the powerful, whereas at Stage IV one sees oneself as a channel to empower others.