The Dow Jones Industrials (DJINDICES:^DJI) have posted amazing gains so far in 2013, prompting many investors to get nervous about its future prospects. But there's one compelling reason investors should be optimistic going into the New Year.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, looks at this key driver of stock market growth for the rest of the year. Dan notes that in order to cut back on stock exposure, investors in taxable accounts have to incur capital gains and pay tax on their profits. If they can wait until January, though, the tax liability doesn't hit until they file their 2014 tax-year returns. Dan concludes that while Caterpillar (NYSE:CAT) and IBM (NYSE:IBM) won't see much impact since they've fallen for the year, Boeing (NYSE:BA) and Nike (NYSE:NKE) could continue to rise because of this phemoneon. Dan warns that seasonal factors are never perfect, but this one could nevertheless be a contributing factor to a year-end rally.