One interesting theory about people is the one by Howard Gardner. He developed the idea of multiple intelligences , which defines intelligence as having the ability to solve or create something of value for a culture or community. According to Gardner, seven categories of intelligence exist (at least at the time of his book, Multiple Intelligences ): linguistic, logical-mathematical, spatial, musical, bodily- kinesthetic , interpersonal, and intrapersonal.
Linguistic intelligence is reflected in using words, e.g., talking, verbalizing, speaking. Logical-mathematical intelligence is reflected in using numbers and logic, e.g., calculating, analyzing, classifying. Spatial intelligence is reflected in using pictures and images, e.g., painting, mapping. Musical intelligence is reflected when developing or conducting rhythms and melodies. Bodily-kinesthetic intelligence is reflected via physical action, e.g., sports as well as the physical act of balancing and sorting. Interpersonal intelligence is reflected through understanding and working with people, e.g., communicating, empathizing, motivating. Intrapersonal intelligence is reflected through the understanding of one's self, e.g., feelings, inner thoughts. 
This theory has profound implications for project managers. By understanding which intelligence is developed in team members , they can assign people to tasks that capitalize on their strengths and avoid assigning them to those that may sap their intelligence. Or, if they assign a task to someone who has not developed the necessary intelligence, they can assign another team member to compensate for the weakness. Or, they can send a person for training to augment his or her skills.
The point is not that project managers can improve a person's intelligence. Chances are it will be impossible . The real point is that project managers can capitalize on people's strengths, compensate for their weaknesses, and take some remedial action.
Another popular theory is the whole brain thinking by Ned Herrmann. His theory is based on the concept that the human brain can be divided into left and right sides. It would be instructive before discussing whole brain thinking to provide an overview of the capabilities of the left and right side of the human brain.
According to contemporary theory, a human brain is divided into two halves , left and right. Each side provides a given set of thinking capabilities.
The left brain is associated with the ability to think logically and sequentially, as well as concretely, rationally, and objectively. People who are left-brain dominant, therefore, are good with numbers, problem solving, and defining goals. The right brain is associated with emotions. People who are right-brain dominant are visual, intuitive, and think metaphorically and spontaneously. 
Ned Herrmann developed a more holistic view of the human brain by taking an integrated perspective of the two halves, calling it "whole brain thinking." Viewing the mind as a circle, he identifies four quadrants of thinking. The first quadrant deals with thinking logically, analytically, and quantitatively. People who fall in this quadrant, for example, think realistically and like to calculate numbers. A second quadrant deals with thinking in an organized, sequential, detailed manner. People in this quadrant, for example, like to establish procedures and develop plans. These two quadrants are closely related, forming the left side of the brain. A third quadrant deals with thinking holistically and intuitively via synthesizing and integrating. People who fall in this quadrant, for example, tend to take risks and are spontaneous . A fourth quadrant involves feelings and emotions. People in this quadrant, for example, tend to be emotional and like to teach. These two quadrants are closely related , forming the right side of the brain. 
Herrmann had some interesting insights that can prove useful to project managers. His research reveals that patterns can be reflected in "spider charts " to indicate a person's dominance in certain quadrants and less so in others. This dominance has implications for a preferred managerial style, chosen vocation, and decision-making style. People who fall in the first (upper left) quadrant are problem solvers. People who fall in the second quadrant (lower left) are planners. People who fall in the third quadrant (upper right) are holistic thinkers. People who fall in the fourth quadrant (lower right) are interpersonal. 
Another one of the more popular approaches for understanding people is the Myers-Briggs Type Indicators. These indicators reflect a person's personality in terms of degree. Based on Jungian psychology, four categories of type indicators exist: Extraversion and Introversion, Sensation and Intuition, Thinking and Feeling, and Perceiving and Judging.
Extraversion and Introversion are the first category. A person in the Extroversion category sees people as a source of energy whereas a person in the Introversion category prefers to be alone. The former is typified through sociability and many relationships; the latter tends towards concentration and fewer relationships.
Sensation and Intuition are the second category. A person in the Sensation category prefers facts and perceives himself or herself as being rooted in the here and now. A person in the Intuition category is imaginative and focuses on the future. The former is typified through emphasis on experience and practicality; the latter tends toward speculation and inspiration.
Thinking and Feeling are the third category. A person in the Thinking category strives to be objective, even impersonal. A person in the Feeling category emphasizes values over facts and does not favor rule-based decision making that the thinking types would prefer. The former is typified through emphasis on analysis and criterion; the latter tends towards subjectivity and persuasion.
Perceiving and Judging are the final category. A person in the Perceiving category tends to resist deadlines whereas a person in the Judging category looks at options, selects one, and sets deadlines. The former is typified by being spontaneous and adaptable; the latter tends towards planning and being fixed. 
According to a popular book on the subject, Please Understand Me by David Keirsey and Marilyn Bates, not everyone is an "either/or" type. Instead, most people fall on continuum. Nevertheless, the four categories of type indicators create sixteen combinations of temperaments INFP, ENFP, INFJ, ENFJ, ISFP, ESFP, ISFJ, ESFJ, INTP, ENTP, INTJ, ENTJ, ISTP, ISTJ, ESTP, and ESTJ.
Interestingly, the authors note that the temperament will have an influence on the way people lead. SP combinations tend to be good negotiators and troubleshooters as leaders . As team members, they tend to be process oriented, concentrating on what is realistic and getting "things in motion." SJ combinations tend to be stabilizers as leaders, setting up regulations and routines. As team members, they are industrious and product oriented. NT combinations tend to be principle oriented, questioning everything on the basis of those principles. As team members, they function as designers or architects , building models. NF combinations tend to be transactional and democratic as leaders. As team members, they are personal in their dealings, being transactional. 
Another popular approach is the Enneagram, a well-received but not scientifically based approach for understanding one's own personality and others. However, this fact should not discredit it because the Enneagram is concerned with normal rather than pathological behavior. It is also easy to apply one's self and others.
The Enneagram identifies nine personality types that are interconnected and reflect dynamic movement from one type to another. The nine are: Perfectionist, Giver, Performer, Tragic Romantic, Observer, Devil's Advocate, Epicure, Boss, and Mediator.
Perfectionists believe that there is only one way to do things; they tend to be critical of others and, sometimes, of themselves as well as being practical and rigid. Givers focus on meeting other people's needs, satisfying their need for affection and approval as well as being demonstrative. Performers are the competitive types who focus on achievement in a comparative manner vis- -vis others; their efficiency and persuasiveness enables them to progress. Tragic Romantics are idealistic, attracted to what may be unrealizable, and tend to be dramatic and creative. Observers like to maintain their distance emotionally and concentrate on keeping their privacy by behaving in a detached manner. Devil's Advocates emphasize thinking over action, more out of fear and doubt; they are constantly questioning and vigilant. Epicures avoid commitment and depth, acting superficial and more like dilettantes. They tend toward optimism but find it difficult to commit to anything. Bosses are the "take charge" kind of people, being protective and combative; although very territorial, they are also very protective of people within that territory. Mediators see all viewpoints, looking for agreeableness among everyone; this requires considerable patience on their part.
The nine personality types are based on nine features of emotional life. These nine features are anger, pride , deceit, envy, greed, fear, gluttony, lust, and sloth, and they play a key role in our motivations and the preoccupation of one prevents people from other activities.
The Enneagram applies to all people because everyone has the potential for all nine types; however, most people identify with only a few. Helen Palmer says that the Enneagram is dynamic, not fixed. 
From a project management perspective, the principal advantage of the Enneagram is that it provides the ability to see reality from various perspectives, e.g., from the "lens" of a Perfectionist or Tragic Romantic. An ancillary advantage is that it helps project managers to become aware of their own biases during planning and execution of their projects. This awareness can help them to obtain a more balanced perspective, especially during decision making.
In her book The Drama of Leadership , Patricia Pitcher develops an approach to look at people from an emotional perspective exhibited through behavior, thought processes, and temperament. Using the research described by Antonio Damasio in Descartes' Error , she identifies three types of people: Artist, Craftsman, and Technocrats.
Artists focus on people, providing imagination and inspiration. They are the visionaries of the three categories. Additional words to describe them are intuitive, emotional, and exciting.
Craftsmen are the experts, rooted in the world of practicality. They "know their stuff" and are hardworking and responsible. Additional words to describe them are trustworthy, well balanced, and open minded.
Technocrats are very analytical, the least emotional of the three types, and tend to be determined and uncompromising . Additional words to describe them are intense , methodical, and detail oriented. 
Each category has their strengths and weaknesses, reflected in the way they lead. Artists provide the vision, but they also can be emotionally unpredictable. Craftsmen, albeit predisposed to structure, tend to be more sensible . Technocrats have the expertise, but tend not to be very personable or compromising . 
An entertaining yet useful way to identify a person's personality is to use the color approach of Taylor Hartman that he discusses in his book, The Color Code .  He provides a four-color typology to identify and understand human behavior that is based on needs and wants. Each color represents a set of strengths and weaknesses, with most people having a primary and a secondary color. The four colors are: red, blue, white, and yellow.
Reds are individuals who are quite independent in thinking and action but do not allow others the same latitude. They want to strive for action and produce results right away. They are motivated by power via technical proficiency.
Blues are individuals who are oriented towards being a "people person." They want to be appreciated and are driven by value and conscience. They are motivated by intimacy via moral goodness.
Whites are individuals who seek tranquility by avoiding confrontation and keeping a low profile. They are motivated by peace , especially internally.
Yellows are individuals who are the "party animals" of the four categories. They desire attention, especially the center of attention. They are motivated by fun via social presence.
Each color has its weaknesses. Reds, while being very active and productive, can be highly critical and impatient. Blues, while passionate about work and people, can be perfectionists and distrustful. Whites, while being diplomatic and tolerant, can be indecisive and noncommunicative. Yellows, while being and radiating enthusiastic, can be undisciplined and superficial. 
From a project leadership perspective, the color code can prove quite useful. There are times in the project life cycle when reds can prove useful, e.g., a project has a slow start, and blues can be brought on for empathetic talents to bring people together at the beginning of a phase. Yellows can come to bring levity to an intense phase, e.g., implementation. Whites can bring the virtue of patience, e.g., when a project begins too quickly without concurrence from principal stakeholders.
The DiSC Model, which has been around for a while, is useful for understanding human behavior through dimensions. DiSC is an acronym for Dominance, Influence, Supportiveness, and Conscientiousness. It is a powerful tool to explain what occurs when a personality of one of the dimensions must deal with its environment. It provides people with the ability to respond with an appropriate dimension to a specific situation. If a situation changes, a person can exhibit a different dimension.
Dominance pertains to directness and assertiveness. It consists of elements like daring, forceful character, and competitiveness . Their strengths are being self-starters, go-getters; those who seek and take action. Their weaknesses include being abrupt, intimidating, and impatient. People with a high dominance "factor" are the hard chargers and tend to prefer to be in charge. They excel when given the autonomy, in a fast-paced environment that requires results.
Influence is for inducing and expressing an interest in other people. It consists of elements like persuasiveness and attractiveness. Their strengths are working with other people and being positive. Their weaknesses include overlooking important details and being sensitive to the behavior of others. People with a high influence factor take selling or persuasive approaches rather than forcing people into compliance. They excel, too, in fast-paced environments that provide recognition and the opportunity to be creative. They excel in environments that are informal and collegial.
Supportiveness is for submissiveness. It consists of elements like willingness and nonaggressiveness. Their strengths include being reliable and practical. Their weaknesses include a rigidity in thinking and slow adaptation to change. People with a high supportiveness are people who proceed cautiously as they study a subject thoroughly.
Conscientiousness is for being compliant. It consists of elements like resignation and harmony. Their strengths include being detail oriented and following procedures. Their weaknesses include being unreceptive to criticism and insecure . People with a high compliance factor emphasize the importance of order and proceed cautiously. They excel in environments with sufficient resources and are conservative. 
The prevalence of a particular dimension will be reflected in the overall style of an organization. A Dominant organization, such as a project, will be decisive , direct, competitive, and action oriented. An Influence organization will be collegial, with plenty of socializing and little discipline in terms of procedures and schedules. A Supportiveness organization is one with little conflict, that emphasizes loyalty and cooperation. A Conscientiousness organization is one that emphasizes detail, formality , and being methodical. 
All of the above approaches to understanding human behavior tell us much about people. What they do not tell us much about is how people respond to adversity and difficult situations. Paul Stoltz  developed the concept of the Adversity Quotient (AQ) to address this issue. According to him, people have different capacities or abilities for dealing with adversity, e.g., setbacks. A person's AQ is reflected in how resilient and optimistic they are when dealing with adversity. A person's AQ is a pattern that has been developed and is exhibited in thought, emotion, and action.
The capacities for dealing with adversity consist of four dimensions: Control, Ownership, Reach, and Endurance. Control deals with a person's response to adversity, either delayed or spontaneous. Ownership is the extent a person feels he or she can improve the situation. Reach is the degree to which adversity is allowed to permeate his or her life. Endurance reflects how he or she perceives adversity and, therefore, is willing to persist through it. An overall AQ score determines one's capacity to deal with adversity. In Adversity Quotient @ Work , Stoltz says that a CORE response to an event reflects one's perception of it. Hence, the higher one's AQ, the greater the likelihood of a person perceiving an event as positive. Likewise, the lower the AQ, the probability increases that a person will view it as negative.
Stoltz identifies three categories of people reflecting their response to adversity using the metaphor of mountaineering: climbers, campers, and quitters. Climbers are people who continue on their ascent, learning along the way, but not turning away. Campers, when faced with adversity, retreat into their comfort zones to feel secure. Quitters cease , even retreat, thus avoiding the negative aspects of an ascent; they simply give up. 
 Howard Gardner, Multiple Intelligences , Basic Books, New York, 1993, pp. 15 “26.
 Jacquelyn Wonder and Priscilla Donovan, Whole Brain Thinking , Ballantine Books, New York, 1984, pp. 3 “19.
 Ned Herrmann, The Whole Brain Book , McGraw-Hill, New York, 1996, pp. 15 and 23.
 Ned Herrmann, The Whole Brain Book , McGraw-Hill, New York, 1996, p. 272.
 David Keirsey and Marilyn Bates, Please Understand Me , Prometheus Nemesis, Del Mar, CA, 1984, pp. 13 “26.
 David Keirsey and Marilyn Bates, Please Understand Me , Prometheus Nemesis, Del Mar, CA, 1984, pp. 129 “166.
 Helen Palmer, The Enneagram , HarperSanFrancisco, San Francisco, 1991, pp. 7 “41.
 Patricia Pitcher, The Drama of Leadership , John Wiley & Sons, New York, 1997, pp. 15 “49.
 Patricia Pitcher, Artists, craftsmen, and technocrats, Training and Development , pp. 30 “33, July 1999.
 Taylor Hartman, The Color Code , Fireside Books, New York, 1998, pp. 43 “124.
 Taylor Hartman, The Color Code , Fireside Books, New York, 1998, pp. 43 “124.
 Taylor Hartman, The Color Code , Fireside Books, New York, 1998, p. 68.
 Tom Ritchey and Alan Axelrod, I'm Stuck, You're Stuck , Berrett-Koehler, San Francisco, 2002, pp. 19 “117.
 Paul G. Stoltz, Adversity Quotient @ Work , William Morrow, New York, 2000, pp. 19 “77.
 Paul G. Stoltz, Adversity Quotient @ Work , William Morrow, New York, 2000, pp. 19 “77.