Facebook (FB), Apple (AAPL), Amazon (AMZN) and Alphabet’s Google (GOOG) investors are happy the four dominate the markets and consumers’ wallets, but their large market shares in the U.S. could harm productivity and growth down the road.

In a new study, Sophie Guilloux-Nefussi, an economist at France’s central bank, found that the combined market share of the eight largest companies in more than 60% of sectors in the U.S. economy rose between 2002 to 2012. The study says this trend toward market concentration helps profits but contributes to a decline in investment and salaries. It is also associated with a drop in the creation of new businesses and jobs, which may hurt growth. (See more: Why Morgan Stanley Favors Dull Utility Stocks Over Techs.)

“If these companies are in a dominant position in their market, they enjoy monopoly rents and have little incentive to invest to increase their productive capacity,” wrote the economist in the report. “This behavior may explain the weakness of investment in recent years, despite high rates of return on capital.” (See more: Facebook and Google’s Days Are Numbered: Soros.)

According to Guilloux-Nefussi, the net investment rate of non-financial companies when compared to profits dipped from 19% on average over the period 1980-2000 to 12% since 2000. 

While corporate profits have grown, salaries have not kept up, causing a rise in inequality. "At the level of each sector, the most successful companies are also generally the least intensive in work. The concentration of production by a few large companies can therefore explain the decline in the share of wages in GDP," said the study.

The study says the decline in investment and rise of profits suggests that the trend toward market concentration is due to "stronger barriers to competition" and "overly permissive application of anti-competitive regulation by US government agencies in recent years" rather than technological progress that favors leaders in each sector.