There’s no escaping the conclusion that the stock market is not a level playing field where all investors, large and small, have an equal shot at a fair deal.

A recent groundbreaking study found that undetected insider trading occurs in a stunning one-fourth of public-company deals. Experts have long debated the pros and cons of high-frequency trading, another pervasive practice, but there is no doubt that it gives superfast traders the jump on others in trading stocks. And the very idea of trading on a public exchange, where stock prices and trading volumes are visible to all, is being eclipsed by private trading of public stocks in off-exchange venues, called dark pools, usually operated by banks.

These are not the only ways in which big market players make money at the expense of other investors. The Senate Permanent Subcommittee on Investigations recently held a hearing on “maker-taker” pricing in which stock exchanges pay rebates to brokers for sending them buy-and-sell orders. The practice has been around for years, as a growing number of exchanges — there are now 11 public exchanges in the United States — have battled for business. What is new is the compelling evidence that the rebates are corrupting.

Under federal trading rules, brokers must provide “best execution,” which usually means finding the best stock prices for clients who pay them to buy and sell shares. But the rules also recognize that for some trades, getting the best price is only one part of best execution; the speed, size and cost of a trade must also be considered.