We ask you to indicate your sex and birthdate (or just birth year) because age and sex are the two pieces of information we must have in order to use your data in scientific studies. We also use your age and sex information to show you age-appropriate faces and ask sex-specific questions (such as about menstrual cycles).
Please leave these blank if you would rather not give this information. You will still be able to participate in experiments, but your data will not contribute towards our studies.