Over-investigation, the practice of ordering diagnostic tests when the potential harms outweigh the expected benefits, has become a pervasive feature of modern medicine. While intended to reduce diagnostic error, this practice often exposes patients to unnecessary risks, including false positives, overdiagnosis, psychological distress, and avoidable downstream procedures, without improving outcomes. This review examines the psychological, cultural, and systemic factors contributing to over-investigation. Key cognitive determinants in both physicians and patients include discomfort with ambiguity, commission bias, and a "medical maximizer" orientation. Defensive medicine, driven by concerns about malpractice, further fuels excessive testing. Patient expectations, professional pressures, and cultural norms reinforce the cycle, while systemic factors, such as time constraints, fee-for-service incentives, and easy access to abundant diagnostic resources, exacerbate the problem. Laboratory process-related issues, including test unreliability and unnecessary repetition, compound the waste. Ultimately, over-investigation diminishes patient trust, increases healthcare costs, and contributes to clinician burnout. Addressing it requires a deliberate cultural shift that emphasizes clinical reasoning, evidence-based guidelines, patient-centered shared decision-making, and systemic reforms that support the thoughtful and judicious use of diagnostic testing.