The fourth-quarter market rout that suddenly turned investors' 2018 gains into losses was a mixed experience for robo-advisors. Slightly more than three-quarters of the robos whose 2018 returns were tracked by The Robo Report finished the year with losses in their balanced portfolios.

The best robo performers for the fourth quarter, based on returns, were FutureAdvisor, which was purchased by BlackRock in late 2015, Ellevest and Merrill Edge. Their losses ranged from 6% (for FutureAdvisor, which lost the least in its equity allocation), to 6.61% for Merrill Edge.

Those losses are slightly less than half the losses in the S&P 500 in 2018, and reflect modest gains in the bond allocations of most robos during the quarter.

Merrill Edge, which had switched to a more conservative portfolio in August, also placed among the top three robo performers for the year, based on returns. Fidelity Go had the best one-year performance (-4.6%) followed by Zacks Advantage (-4.7%) and Merrill Edge (-4.9%).

Both Fidelity Go and Zacks Advantage allocated less than a third of their equity portfolios to international stocks, which was less than many other robos and helped their performance for the year, according to The Robo Report.

Fidelity Go was also the best-performing robo over the two years ended Dec. 31, 2018, based on annualized returns, reflecting the highest gains for equity and bond allocations. Its equity portfolio focused on domestic large-cap stocks; its bond portfolio, on municipal bonds.

The Robo Report previously rated Fidelity Go the best overall robo-advisor as a result of its relatively strong performance, low all-in 35-basis-point management fee, planning tools, easy navigation and no minimum investment.

For the first time, The Robo Report published three-year rankings of robos. Among this small group of seven, SigFig had the best performance, returning 6% on an annualized basis, followed by Schwab, up 5.9%, and WiseBanyan, up 5.5%.

SigFig's performance was boosted by an overweight holding in emerging market equities as well as emerging market sovereign debt in its bond portfolio.

Schwab also benefited from holdings in emerging market equities along with large-cap stocks, U.S. high-yield corporates and international bonds, according to the report. WiseBanyan, like fourth-place Vanguard, had broad-based holdings, plus an allocation to short-term high-yield bonds, which helped boost returns.

Another notable update in the latest report is the increasing number of robos that are offering socially responsible investment (SRI) portfolios. They include Betterment, Ellevest, Morgan Stanley, TD Ameritrade, TIAA and WealthSimple. Morgan Stanley and TIAA also have separate "active" versions that either invest in actively managed funds or are directed by an active manager.