The idea that, because the human population increases exponentially while the planet's resources only increase linearly, population control is inevitable.
“While every individual has ample incentive to increase their gains, they have little incentive to preserve the resource for the good of others.”
This theory pushed the narrative that unregulated communal resources have no choice but to be overexploited and destroyed.
Central assumption of human beings being selfish, and rational actors at the same time Hardin, who pushed this theory out in his work, was a white nationalist himself (as recognized by the Southern Poverty Law Center). His beliefs on anti-immigration especially on non-white immigrants, support for forced sterilization and staunch opposition for social welfare programs are reflected in his work in environmentalism as well.
Reality?
Economist Elinor Ostrom who was awarded the Nobel Memorial Prize in Economic Sciences showed that people can, and do, regulate the use of the commons to prevent their overuse. In fact, in the absence of government regulation, communities usually come up with their own systems to regulate commons that outperformed governmental regulations.