From Vox:

Since you asked (many times).

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I thought I would explain, once and for all, why I hardly ever talk about population, and why I’m unlikely to in the future. …

When political movements or leaders adopt population control as a central concern … let’s just say it never goes well. In In practice, where you find concern over “population,” you very often find racism, xenophobia, or eugenics lurking in the wings. …

History is replete with examples, but perhaps the most germane recent episode was less than 20 years ago, at the Sierra Club, which was riven by divisions over immigration. A group of grassroots members, with some help from powerful funders, attempted to take over the national organization.

These members advocated sharply restricting immigration, saying the US should be reducing rather than increasing its population. …

The Sierra Club won that fight, and the “green anti-immigrant” movement has mostly been driven to the fringes, but conservative media is still getting ratings out of it. If you can stomach it, watch this entire segment with Tucker Carlson of Fox News — it hits all the usual notes, culminating in an interview with some professor who wrote a book about reducing immigration for environmental reasons.