Congress has successfully engineered lower tax rates for long-term investors -- but amid all the haranguing over the new law, few have questioned why the measure was necessary.

Because lower tax rates are almost always good news for investors, few people have noticed that the original justifications for lowering rates no longer apply.

The tax code hasn't always included a tax break for investors who hold stock for a certain length of time. In fact, in the first mention of a tax on capital gains (in the law in effect from 1913 to 1921), the rate was no different from marginal income tax rates -- which at that time topped out at a whopping 73%.

Eventually, though, Congress decided that encouraging investors to hold on to their investments a bit longer would be good for several reasons, most of which are no longer driving reasons for today's tax cuts.

"What has historically been the case for lower long-term rates has been so distorted over time, it's hard to discern any reason for them now," says Tom Oschenschlager, a partner with Grant Thornton in Washington, D.C.

Originally, the reason to tax the gain on securities held for a while was that a good portion of any appreciation would actually be attributable to inflation, and therefore shouldn't be taxed.

"With inflation near zero these days, though, that's hardly the case anymore," says Mark Luscombe, a tax attorney with the research firm CCH. Also, Luscombe notes, the holding period in the early life of the reduced capital-gains tax was 10 years -- a mind-boggling amount of time to hold an investment by today's standards. These days, investors need to hold a security for just 12 months in order to qualify for the long-term capital-gains rate of 15%. Even in times of high inflation, profits from the sale of a stock held less than a year can hardly be attributed to inflation.

But while the long-term capital-gains rate has vacillated between 12.5% and 35%, the holding periods required to qualify for the advantageous rates dropped sharply and stayed down -- ranging from six months to two years. With holding periods that short, inflation is scarcely an issue.

Instead, over the years, Congress has adopted additional theories to support its inability to make the holding periods more stringent or increase rates. Providing investors with a financial incentive to stay put rewards them for putting their money at risk, and that can be good for the economy. It provides companies with a more stable pool of capital from which to work. Think of it as similar to a magazine subscription: Publishers offer big discounts to subscribers because it's worth it to them to guarantee the financial commitment.

And while a year is a fairly arbitrary amount to require before granting a tax break on profits from a sale, there is some sense to it, says Walter Hellerstein, a tax law professor at the University of Georgia law school. "If you buy a stock in January and sell it in August, any profits would look more like ordinary earned income and should be taxed as such," he says. "But for every argument, there's a counterargument."

Some things, it seems, really don't change.

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