This paper gives a brief overview of the evolutionary history of human diet,discusses various reasons why some modern humans choose to abstain from eating animal products,and explores the psychological effects occasioned by the eating of meat.The authors argue that when modern,scientifically literate humans choose to eat meat and animal-milk-based products,while being simultaneously aware of the associated poor outcomes in terms of personal health,environmental sustainability,and animal suffering,then psychological stress is an inevitable outcome.They highlight some key recent additions to the growing body of evidence which demonstrates that the choice to eat meat is a factor in environmental destruction,animal misery,and personal health risks,and explore the causes and consequences of the predictable psychological stresses that result for those individuals who continue to habituallv eat meat.