Under the European rule, Mr. Jackson said in an interview, “consumers can see how much of their commissions are going to research.” The rule “also makes it easier to monitor best execution,” and it could encourage the creation of high-quality independent research shops, he said.

But greater transparency is not something Wall Street generally wishes for, which might explain why some firms are opposing an unbundling of research and trading costs.

Another reason Wall Street prefers the status quo on research and trading is the information edge it provides. Big investor orders represent crucial information for brokerage firms, which can generate rich profits by trading around the transactions. If investors start sending orders to firms based only on superior execution, the big broker-dealers could lose that information source.

Under the European system, research and trading are likely to improve, investors say. That’s because they’ll be able to reward firms that produce meaningful investment analysis as well as those that generate excellence and efficiency in trade executions.

Investors’ costs will also come down because they will no longer be at risk of paying up for bad executions or mediocre research. “Separately shopping for research and trading will significantly reduce investors’ costs,” said Tyler Gellasch, executive director of the Healthy Markets Association, a nonprofit organization focused on improving the integrity of the nation’s financial markets. “That directly translates to higher returns and more money for retirees and college savings funds.”

Even as Europe moves to a new and fairer regime, some on Wall Street are quietly urging the Securities and Exchange Commission to maintain the bundling of research and trading in American markets. Among them is Goldman Sachs, which has told some of its big investor clients that it opposes separating research from trading. A spokesman for Goldman declined to comment.

The main argument offered against separating research and trading is that broker-dealers that publish analysts’ reports would have to register as investment advisers, subjecting them to an additional regulatory burden.