The slogan was made popular by Sally Miller Gearhart.
https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Sally_Miller_Gearhart.html
> She has been controversial for her statement that "The proportion of men must be reduced to and maintained at approximately 10% of the human race", made in her essay "The Future-If There is One-Is Female".
I'm guessing you're talking about this book?
If you do this, keep it in one post.
edit: Also, since you didn't post the original quote and I just remembered that rule exists, I'm gonna have to remove this submission. PM the mods if you add the original quote, as an image or in the self-text or whatever.