Alternatively smart people and/or hard working people make more money, they also tend to take "hard" courses such as advanced math while in highschool.
Yeah, it's hard to figure out from that article how the study was done. Unless it's a true randomized study where people were randomly assigned to take either more or less math, it's hard to know how much of that is self selection.
Individual effort is controlled for in the study discussed: "each extra required maths course raised the annual income of black males by 15%". Of course, this study still doesn't make a strong case for causation -- the number of required math courses in a school is likely correlated with school quality, which is likely correlated with other socioeconomic factors.
Huh? How do you infer that individual effort was controlled for by quoting "each extra required maths course raised the annual income of black males by 15%"?
No causation required.