human 42 major clinical study motivation female 38 male 36 Humans 34 adult 33 Motivation 26 Article 24 controlled study 21 Adult 20 Female 19 Male 19 psychology 17 article 16 middle aged 16 young adult 15 Middle Aged 14 human experiment 12 questionnaire 12 cross-sectional study 11 Young Adult 10 adolescent 10 aged 10 Adolescent 9 Aged 8 Cross-Sectional Studies 8 Surveys and Questionnaires 8 child 7 physiology 7