This is a variation of the Mythical Man Month, and probably equally unintuitive. However, the research in this area is bulletproof and has been verified thousands of times by independent researchers, and its conclusion is “Adding more developers to an already late project only makes it later.”
The reasons of course includes that you’re forcing your only productive resource to spend precious time teaching the new developers to understand the existing codebase, in addition to that now the project requires overhead, coordination, and collaboration, adding to its overhead – So while you believe that you’ll be able to finish faster, the exact opposite is the result.
If your job is to move 1000 bricks from A to B, and you’re late, adding more manpower to the job obviously results in finishing earlier. Hence, human experience have taught us that adding more manpower to a job, results in finishing the job faster. Therefor, when a manager is faced with the prospect of not finishing the software project in time, his intuition is to add more manpower to the job. However, you’ll only make it later. Understanding this though, and believing in it, requires huge amounts of experience – Experience most software development managers don’t have. In addition it requires courage, courage to say “NO” when your investors and CEO are telling you to add more resources to your team. However, a mature and experienced software development leader will easily be able to explain this fact to his managers, allowing for the project to finish as fast as possible, without adding to the cognitive load of his team, by adding more manpower to an already late project.
There exists many reasons to hire more manpower, but making the project faster, is not one of those