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Lower socioeconomic status is thought to add stress, often in the form of less time for studying because teenagers at the lower end of the socioeconomic ladder have to work more. They also can't afford tutors, have fewer books in the house, etc.

Yes, everyone can study for it, but generally the easiest ways to boost your SAT scores are to pay a tutor to drill you through the parts that can have the biggest impact on your scores, and to take the test multiple times until you get a score you like, both of which can get very expensive. Retaking the test is really helpful, as you'll be more familiar with the format and less stressed out the second time around, but a lot of students don't have the opportunity to take it more than once.

Allegedly, with time, money, and tutors you can "study to the test" pretty effectively.