Yes, correlation doesn’t equal causation. But there’s more nuance to relationships than that - we can look at whether or not something is a LIKELY cause of something else, or whether it’s a factor along the causal pathway, or whether it’s a third variable that is very strongly tied to the original cause. In this case, high school GPA is probably all of the above.
There are a lot of causes of high high school GPAs - hard-working, organized, and intelligent students, of course, are more likely to get higher high school GPAs. They are also more likely to do well and be diligent in college and in their job searches and beyond, setting them up for higher pay down the road.
But the researchers also controlled for “innate ability.” While it’s really difficult to fully adjust statistical models for something as nebulous as “innate ability,” that does mean that on some level they found that high school GPA is still related to earnings regardless of how “smart” a given student is. That probably goes back to the diligence/organization part - even if a given student is less innately talented/smart than a similar student, if they work hard enough to get a good high school GPA they’ll still probably do better later on - because their hard work more or less “makes up” for the lack of innate talent. That’s not brand-new information, either; there’s a lot of educational research that shows that hard work can be just as if not more important than any innate talent or intelligence someone has.
They also controlled for a variety of economic and parental factors to try to look at the relationship regardless of parents’ income, socioeconomic status, neighborhood, parental education, etc. So yes, if one’s parents are more highly educated one probably will do better in high school, but they tried to select out for that. This relationship persists even when the contribution of those factors are deleted from the analysis.
The researchers also found something else interesting out.
The correlation between high school grades and overall educational attainment was strongest for minorities. African American and Hispanic men were actually more likely to go to college and graduate school than whites with similar GPAs, says French, a sign that minority students with good grades may be more motivated to finish school and advance their education. However, those higher grades in high school didn’t translate into higher earnings for African Americans, which French says could be due to a gap in the opportunities made available to minorities.