I've seen well-functioning teams suddenly struggle just because an *extra* developer was added to the team.

Hence, a rule: You can't add or remove people from a team. Any people change creates a *new* team, with new dynamics.

And, adding more people to a project slows things down in the short-term (since others need to help them get up to speed).

It typically reduces per developer output over the long-term too (due to communication and coordination overhead).

For more, see "The Mythical Man Month"

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