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Is a “total stock market” index fund diverse enough alone?
You're missing the concept of systemic risk, which is the risk of the entire market or an entire asset class. Diversification is about achieving a balance between risk and return that's appropriate for you. Your investment in Vanguard's fund, although diversified between many public companies, is still restricted to one asset class in one country. Yes, you lower your risk by investing in all of these companies, but you don't erase it entirely.  Clearly, there is still risk, despite your diversification. You may decide that you want other investments or a different asset allocation that reduce the overall risk of your portfolio. Over the long run, you may earn a high level of return, but never forget that there is still risk involved. bonds seem pretty worthless, at least until I retire According to your profile, you're about my age. Our cohort will probably begin retiring sometime around 2050 or later, and no one knows what the bond market will look like over the next 40 years. We may have forecasts for the next few years, but not for almost four decades. Writing off an entire asset class for almost four decades doesn't seem like a good idea. Also, bonds are like equity, and all other asset classes, in that there are different levels of risk within the asset class too. When calculating the overall risk/return profile of my portfolio, I certainly don't consider Treasuries as the same risk level as corporate bonds or high-yield (or junk) bonds from abroad.  Depending on your risk preferences, you may find that an asset allocation that includes US and/or international bonds/fixed-income, international equities, real-estate, and cash (to make rebalancing your asset allocation easier) reduces your risk to levels you're willing to tolerate, while still allowing you to achieve returns during periods where one asset class, e.g. equities, is losing value or performing below your expectations.