"Do what you are good at" commits the hidden-benefit fallacy

MarginalRevolution.com has another great post using marginal analysis to explain why boys are more likely than girls to major in STEM subjects--because children are told "do what you are good at!"

•Girls get As in History and English and B’s in Science and Math which implies that (MCMath > MCEnglish) so they study English and History

•Boys get B’s in Science and Math and C’s in history and English (MCMath < MCEnglish) so they study Math.  

This advice is incomplete as it commits the hidden cost fallacy by ignoring the benefits of studying Math.  The correct advice is "do what profits you the most!"  If the marginal profitability of studying Math is bigger than the marginal profitability of studying English, study Math!

•If (MRMath-MCMath) > (MREnglish-MCEnglish), then a child should study Math, not English.  

...stop telling people to do what they are good at and instead tell them to do what pays! STEM fields pay more than the humanities so if people were to follow this advice, more women would enter STEM fields.