Conventional wisdom in tort law maintains that the prevention of undesirable risks mandates restriction of harmful conduct. Against this widely held conviction, this Article shows that undesirable risks often stem from insufficient, rather than excessive, activity. Because negligence requires investments in only cost-justified care, parties might deliberately limit their activity so that the size of the ensuing risk would be lower than the cost of welfare-enhancing precautions. Parties\u27 incentives to strategically restrict their activity levels have striking implications for the inducement of efficient harm prevention. The overlooked paradigm of insufficient activity calls for the imposition of a new form of tort liability, justifies the...