<prompt> Human: Im fairly naïve about investing. Since I started my private retirement fund, Ive had a Roth IRA with Fidelity, all invested in a single fund: Parnassus Core Equity Investor PRBLX. It has performed pretty well for me, but Im concerned Im not diversified enough.As a result, Ive been looking at robo-advisors, and I now have another Roth IRA with a robo-investor. Ive been reasonably happy with them, and I had decided to transfer my funds out of Fidelity into the robo-investor. Thats when I compared the two side-by-side.My returns this year on the mutual fund are nearly double that of the robo-investor. This almost makes me want to pull the money out of the robo-investor and dump it all into the mutual fund instead.What should I do in this scenario? Is the lack of diversity from having all the money in a single fund going to bite me in the long run even though it seemingly hasnt over the past 10 years? Should I listen only to the numbers and move everything to the mutual fund?  <response>: all invested in a single fund: Parnassus Core Equity Investor PRBLXI looked into this fund. It has a high expense ratio, which often correlates with lower returns. Its all stock, mostly US domestic, but it does not meet the expectations of a US domestic stock fund over the last year. For example, Vanguard Total Stock Index VTSMX has 15.98 return over the last year, and Vanguard 500 Index VFINX has 15.89 over the last year. A more diversified fund often recommended here, Vanguard Target Retirement 2055 VFFVX is up 15.29 over last year.I dont know what robo-advisor you used, but its odd that it has only half of your Parnassus fund. Perhaps you used a conservative investment option, whereas Parnassus, like any 100 stock fund, is pretty aggressive. What should I do in this scenario?I would suggest neither of the options youve done so far. Instead look around here regarding rpersonalfinancewikiinvesting, index funds, expense ratio, 3 Fund Portfolio and target date funds.Typically when someone here asks us what should I invest in? we suggest Vanguard Target Date funds. Its investing on easy mode, and will have pretty good returns over time. Target Date funds are well diversified, and typically they include all of the diversification that you need. Should I listen only to the numbers and move everything to the mutual fund?Past performance does not necessarily mean future gains. The examples I gave you have better returns over the last year, but its more important to understand why they have better returns. In this case its because they are index funds with low expense ratios and they dont try to play the market, they just allow the market to increase their value. 