... a development team is coming together, and Birds of a Feather tend to Flock Together.
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Homogeneous teams that comprise too many of the same kind of people easily fall into groupthink-like dysfunction.
Design is the act of making change to the world. In software, it usually means changing the literature of an author who came before and encoded a solution in a programming language. That author usually remains as part of the community that retains an interest in the code (see CodeOwnership, and the combination of ConwaysLaw and OrganizationFollowsMarket (which implies that architecture follows market).
Change is a process that has several phases, starting with complacency, which is upset by an opportunity or realization of an oversight. There is a struggle to identify solutions, the process of realizing the solution, culminating in deployment.
Different people are more comfortable with some parts of this process than with others. Some people are good at identifying problems, others with the innovative processes of identifying solutions. Yet others are good at focusing on implementation. The variance in comfort comes from variance in experience and individual background and temperament. This is true even when programmers in their role as designers, making a change, are the same programmers who were the original authors of the code.
Therefore:
Consider temperaments and diverse experience backgrounds when assembling a team. This diversity sometimes lines up with social classifications like age and gender, but more generally can be assessed on a personal level.
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One source of variation is the variety of domains in the application itself; see DomainExpertiseInRoles. There is an open question whether a SelfSelectingTeam prejudices a homogeneous group outcome; in any case, DiverseGroups can be a good audit of a SelfSelectingTeam.
Another source of variation is the variation in roles. In a vestigial pattern DiversityOfMembership, the pattern recommends building a requirements teams from diverse roles:
The team should include a developer, a user or user's representative, and a system tester (at least one of each). These individuals will work through the issues surrounding product requirements, often using small prototypes to identify the requirements and determine testing criteria. The user of prototypes can be closely tied to using use cases or similar usage scenarios as analysis and validation tools.
One area in which this approach is especially useful is in the specification and design of the user interface. The developer creates mock-ups of the user interface, and the user and system tester examine them. In this way, this small subteam can go through many different designs of the user interface and select the best one.
See more about this kind of diversity in HolisticDiversity.
Sometimes teams form around mutual interest and talent in a domain, as in SubsystemBySkill, and diversity falls along other dimensions of interest.
Yet another kind of diversity is ethnic diversity. The oft-touted value of ethnic diversity is that it brings together people who can think about problems differently, from much different perspectives, which improves the chance of finding a good solution. But there are other subtle advantages. In a large multinational corporation, we found that each department had a few members of French national origin. The French all ate lunch together, which provided a natural path of communication flow between departments.
One kind of person you want in the mix is a PublicCharacter.
However, one must guard against stereotyping people; e.g., using personality instruments or other information to limit the roles of people in organizations. See [BibRef-KerthCoplienWeinberg1998].
See the related pattern BalancedTeam in [BibRef-Bramble2002].