The evolutionary stability of sex determination systems varies dramatically across species, with some lineages maintaining the same chromosomal sex determination for millions of years while others switch frequently. This fundamental biological puzzle has implications for understanding genetic stability, reproductive fitness, and the evolutionary constraints that shape complex organisms. New theoretical modeling reveals how stabilizing selection—the evolutionary pressure that maintains optimal trait values—creates unexpected genetic barriers that lock in sex chromosome systems once they become established. The research demonstrates that when multiple genes contribute to sex determination (polygenic control), stabilizing selection causes chromosome-specific genetic drift patterns that make transitions to new sex-determining mechanisms increasingly difficult over time. This polygenic architecture essentially builds evolutionary inertia into the system, explaining why some taxonomic groups show remarkable consistency in their sex determination methods across vast evolutionary timescales. The mathematical framework shows that even small selective pressures favoring existing sex determination can compound across multiple loci to create formidable barriers to change. From a broader evolutionary biology perspective, this work illuminates a previously underappreciated mechanism by which complex genetic systems become evolutionarily entrenched. The findings suggest that polygenic traits under stabilizing selection may be far more evolutionarily stable than previously recognized, with implications extending beyond sex determination to other complex biological systems. For understanding human genetics and health, this research provides insight into why certain fundamental biological systems remain remarkably conserved despite millions of years of evolutionary pressure, potentially explaining the stability of critical physiological processes that underpin health and longevity.