Clare Llewellyn, PhD, is a chartered psychologist and associate professor of obesity at University College London, where she leads the Obesity Research Group and the Gemini twin study. In 2011 she completed her PhD at UCL on the nature and nurture of eating behavior and weight in early life and she has a long-standing fascination with the topic, which probably stems from having been a notoriously fussy eater as a child. A decade ago she helped establish Gemini, the largest twin study ever set up to explore the nature and nurture of eating behavior from the beginning of life. She has published nearly 100 scientific papers, articles and book chapters on this topic and has given over sixty invited talks worldwide at international organizations, such as the American Dietetic Association, the UK Royal Society of Medicine and the European Society for Paediatric Gastroenterology Hepatology and Nutrition. She lives in London with her partner, Andy. Hayley Syrad, PhD, is a chartered psychologist. She gained a first-class psychology bachelor's degree at the University of Southampton in 2007 and a PhD in behavioral nutrition at the Health Behaviour Research Centre, University College London, in 2016. Her research has focused on the factors influencing what and how young children eat. She has used real-world dietary data from the largest twin study in the UK (Gemini) to explore children's eating behaviors and has specifically examined the role of appetite and parental feeding practices. She uncovered the finding that children who are more food responsive tend to eat more often, and children with lower sensitivity to satiety tend to eat larger portions. Her research also showed that the portion size served to children can influence how much they consume (larger servings = more consumed), and she was the first researcher to provide evidence of this relationship. Hayley has published a number of articles on infant and toddler feeding.