MSCI's gauge of stocks across the globe <.MIWO00000PUS> is currently at its highest levels ever, up about 17 percent since the beginning of the year with exchanges in Europe, the U.S. and Asia, hitting a series of record highs.

"We expect stretched valuations and rising bond yields to limit equity index performances in 2018 and the prospect of a U.S. economic slowdown in 2020 to further cramp returns in 2019," the French bank wrote in its report.

Socgen's analysts believe that American markets have already priced in the expected economic boost of the tax cuts the U.S. government is currently trying to implement.

"Our valuation model suggests that upside on the S&P 500 <.SPX> is limited: the US equity market is already pricing in a rebound in growth and inflation", they wrote. The index is trading at 22.8 times future earnings.

The fact the U.S. Federal Reserve is widely expected to gradually raise interest rates after years of aggressively expanding its balance sheet is also expected to make stocks less attractive in comparison to bonds.

Turning to Europe, the French bank acknowledges the euro zone's economic recovery is in full swing but argues that current valuations don't leave "much meat on the bone" and that the expected rise in the single currency could also weigh, notably on exporters.

Additionally, with the European Central Bank set to progressively unwind its stimulus package, investors are increasingly wary of the amount of debt some companies have accumulated thanks to historically low interest rates.

Telecoms and cable group Altice (>> Altice), whose shares have collapsed close to 55 percent in the last 30 days due to concerns on its 50 billion euros pile of debt, is an example of what is likely to come, Societe Generale said.

Analysts at the bank saw pockets of growth in Germany, France and in sectors such as financials, but warned that political risks are still present, notably in Spain with the Catalonia crisis and Italy which faces general elections in 2018.

"We also recommend staying away from the UK as Brexit negotiations are accelerating and several scenarios are possible: only a soft Brexit would be supportive for the FTSE 100 <.FTSE>," they added.

(Reporting by Julien Ponthus; Editing by Keith Weir)

By Julien Ponthus

Stocks treated in this article : Société Générale, Altice