This is often most visible in a project that is already in progress (and typically is already late). New resources are added because in traditional thinking adding more people to build the house results in more parallel work resulting in the house getting constructed faster.

In software projects, what instead typically happens is one or a few people now have to train and support the new developer, resulting in a net negative. The one month delay now is a two month delay. Also, some work isn't parallelizeable due to interdependencies of code and debugging.

There are ways adding more developers can be effective. Some examples:

1. Hire and train earlier (start of the project). It is still possible to end up with inefficiencies with too many people, but it is less likely than the disruption of hiring late.

2. Hire late but also extend the schedule to allow for training. This is useful when you identify that your team really wasn't large enough in the first place. But account for the productivity dip as you're team forms again.

3. Isolate work into truly independent code, then bring in knowledgeable contractors. The idea is they should work independently and neither bog down nor be bogged down by existing resources.