"The authors" are a dude and his wife who have been obsessed with the idea for 50 years. Most of what they predicted did not come to pass (of which Erlich later says "we clearly stated that they were not predictions", which sounds weird when you read the first sentence of the book). He has been debunked in multiple ways, and nobody takes him seriously, even if overpopulation is a major problem. Frankly he's just biased and has been trying really hard for a long time to find a narrative to fit his views. https://en.wikipedia.org/wiki/Paul_R._Ehrlich