No study just asks people what they were doing 40 years ago. There are set questionnaires that are filled out on some short period, weekly, monthly, etc... so

No study just asks people what they were doing 40 years ago. There are set questionnaires that are filled out on some short period, weekly, monthly, etc... so there is a lot of data. Self reporting has problems. But this is more to get broad trends, such that more detailed studies can zero in on any interesting findings.