adult motivation human 115 female 94 male 94 Humans 93 Motivation 70 Female 64 Adult 63 Male 56 controlled study 51 Article 49 young adult 47 article 45 Young Adult 38 psychology 38 major clinical study 33 human experiment 32 middle aged 32 Middle Aged 29 adolescent 28 Adolescent 27 physiology 27 questionnaire 24 reward 21 Reward 15 cross-sectional study 15 Surveys and Questionnaires 14 aged 13 clinical article 13