With an ever-smaller pool of people potentially subject to gift and estate tax, advisors seeking business may need to look west.

Montana, Wyoming and the Dakotas have seen the biggest increase in high earners in the past decade. Alaska and Oklahoma also fit the pattern of the fastest-paced growth in wealth shifting from the east.

The Tax Foundation detailed the state trends last month in a report on the growth in taxpayers who earned more than $200,000 a year between 1999 and 2009, the first map of this kind from the nonpartisan research group. While the study did not track actual millionaires, estate planners are "interested in knowing where the high earners are," says Dana G. Fitzsimons, a partner at McGuireWoods in Richmond, Va.

By the end of 2012, the $5 million estate tax exemption ($10 million for couples), $5 million gift tax exemption and 35% estate tax rate will expire. Advisors are telling clients to plan for two possibilities: an extension of the current rules or the return to the $1 million exemption and to higher tax rates.

The rising estate tax exemption has meant that fewer people needed estate planning. Estate tax returns filed dropped around 69% between 2001 and 2009, from roughly 108,000 to 33,500, according to Joseph Rosenberg of the Tax Policy Center. Based on Internal Revenue Service data, in 2013, about 100,000 people will die with an estate greater than $1 million and would be subject to the full tax (assuming no planning), under a $1 million exemption rule, estimates Dennis I. Belcher, also a partner at McGuireWoods. Under a $5 million exemption, the pool of potential clients would be 90% smaller, he says.

The top 10 states for growth in high earners were North Dakota, Alaska, Virginia, Maryland, Oklahoma, South Dakota, Louisiana, Wyoming, West Virginia and Montana. Michigan, Florida, Georgia, Nevada, Ohio, Indiana, Oregon, Wisconsin, Idaho and Arizona were at the bottom.

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