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Is investing exlusively in a small-cap index fund a wise investment?
In a word, no. Diversification is the first rule of investing.  Your plan has poor diversification because it ignores most of the economy (large cap stocks).  This means for the expected return your portfolio would get, you would bear an unnecessarily large amount of risk.  Large cap and small cap stocks take turns outperforming each other.  If you hold both, you have a safer portfolio because one will perform well while the other performs poorly.  You will also likely want some exposure to the bond market. A simple and diversified portfolio would be a total market index fund and a total bond market fund.  Something like 60% in the equity and 40% in the bonds would be reasonable.  You may also want international exposure and maybe exposure to real estate via a REIT fund. You have expressed some risk-aversion in your post.  The way to handle that is to take some of your money and keep it in your cash account and the rest into the diversified portfolio. Remember, when people add more and more asset classes (large cap, international, bonds, etc.) they are not increasing the risk of their portfolio, they are reducing it via diversification.  The way to reduce it even more (after you have diversified) is to keep a larger proportion of it in a savings account or other guaranteed investment. BTW, your P2P lender investment seems like a great idea to me, but 60% of your money in it sounds like a lot.