+1000. I'm running into this now on the project I'm on. My boss understands this. Project management types don't. The best way I've heard this phrased "just because it takes one woman nine months to make a baby does not mean that nine women can do in one month".

Seems like good advice and a good resource; however, your answer doesn't seem to directly address the question of 'how to approach the boss' who doesn't understand this. I.e. would you drop a copy of this book on their desk?

@Time4Tea It's simple enough for a boss to comprehend, although some of the book is obsolescent (very few people care about old-style batch and time-sharing programming any more), and there's a few stinkers in there (the biggest one is his idea to not do information hiding). Still, there's a lot in there that is true but not common knowledge.

Brooks goes into another reason adding people makes things worse -- they add communication overhead. With two people, there's one communication path, with seven people there are eleven, with eleven people there are 55!

@RobCrawford: For 7 people, shouldn't that be 21 instead of 11? (ie. n*(n-1)/2, the number of edges in a complete graph?)

FWIW this is seems to be related to Ahmdal's law.

@RobCrawford It actually gets worse. Each time you communicate, you reset the period of uninterrupted work. If a senior developer gets asked just 1 question every 30 minutes (only 16 questions in 8 hours), he effectively have done 0 hours of complex work. Software development have problems that require hours of uninterrupted thinking time. Having that interrupted means these complex tasks are done incorrectly.

While I agree with Brooks, I've not found this line of argumentation to be helpful in the real world. A lot of this depends on the person you are explaining it to. I've had the experience of explaining why making people work 60 hours a week over a long duration and adding people will only make things worse to have the response "I agree with you but we have to try!"

@JimmyJames I agree and the problem is some statement put by some big name is difficult to simply accept when it's contradictory to expectation of a person we want to persuade. It's even worse when the big name tells nothing the person we're persuading. That's why I've added my answer enabling a mental exercise that should lead to the same conclusion. Combine the two answers together and you have the strategy for a discussion with the manager.

@JimmyB That baby analogy is a bad one. The number of women doesn't have any effect on the time needed. The number of programmers on the other hand does or at least should have an effect. Two organised and communicating developers may finish in almost half the time. Adding a second woman to a pregnancy does exactly nothing. Apples and oranges.

@JimmyB - like the quote. It should make the point clearly to the sort of manager the OP is suffering!

@BlackJack - somehow the point seems to have passed by.

This is out of scope of the question, but I'm very curious—if this principle is correct in practice, why are open source software projects with thousands of contributors (often) so successful?

Apologies for getting the number of connections wrong. I was cribbing from a site discussing Brooks; couldn't remember the formula myself. @Nelson -- I'm living that right now. Tried to explain it, but "those interruptions are important".

@JimmyJames hit the nail on the head there. The management-type who wants to add resources is trying to get something done within a set of constraints. The conclusion to draw from being presented with Brooks' Law is that you can't get that thing done. This is not an easy fact to accept for someone who already feels that they lack control of the situation.

@TomW One thing a manager can control in this situation is minimize damage. Communicate that the project won't finish on time and make sure that the stakeholders adjust their planning accordingly. Many managers avoid this, because it makes them look bad in front of their peers. But leaving them in the darkness about the real status of the project will make them look even worse when the deadline arrives.

@JimmyB I've heard the baby analogy before, and there's a bit of a loophole of logic. Even though nine women cannot make one baby in one month, they can make nine babies in nine months, which averages to the same thing.

@JimmyB Though if I think about that a bit more, that's actually a good way to explain that if all the women were in on the job from the beginning, then they end up contributing to the total more effectively than if they were just thrown in at the last minute.

Make that Brooks’ Law not Brook’s Law.