If you believe in historical data, the safest portfolio is 60% stock and 40% bonds. Since 1928, a person with that allocation could have withdrawn 4.5% of their initial savings every year for 30 years and not run out of money. For a portfolio of 40% stocks and 60% bonds, that number drops to 3.9%. Even though there is not much of a difference in the average yearly return of a 60/40 and 40/60 portfolio, investment returns follow a power law. Over a 30 year retirement, seemingly small differences in yearly returns add up to a large difference in outcomes.