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I don't understand the bolded sentence beneath from Cory Simon's BSc Ohio, PhD UBC old blog. How's negative correlation in the red triangle $<$ (to wit, 'more severe') negative correlation in green triangle?

From this natural compromising behavior in her dating criterion, many of the best-looking guys that this girl dates are not so nice; many of the nicest guys she dates are not as good-looking. By restricting herself to this set of guys, she sees a negative correlation between looks and niceness, despite these two variables being independent in the population! This is Berkson's paradox, and now you can see that this induced correlation stems from selection bias.

We can go one step further: maybe the guys in the very top-right corner (red points) are so nice and handsome that they will not consider dating the girl we are considering, who is just decently nice and good-looking.

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Now her dating pool is even more restricted due to selection bias, and the negative correlation between good looks and niceness is even more severe.

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    It's not that the negative correlation in the red triangle < the negative correlation in the green band, it's that the negative correlation in the green band > the negative correlation in (the green band + the red triangle.) The girl can't access the red-triangle guys (in the one-step-further scenario) because they won't date her. – jbowman Apr 12 '20 at 23:13
  • @jbowman Thanks. Would you like to answer so that I can upvote you? –  May 30 '20 at 04:23
  • Sure! Let me expand upon the comment a little in the answer. Thanks! – jbowman May 31 '20 at 20:57

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