Seattle Mayor Ed Murray recently announced a plan to raise the minimum wage in his city to $15/hour over the next few years. The plan emerged from a special business/labor advisory committee, approved by 21 out of 24 its members, after four months of hearings, academic studies, and debate . The measure awaits approval by the City Council, but the move to $15/hour in Seattle seems well underway.

Seattle may be the first, but it won’t be the last, city to take this bold step. Granted, there were some peculiarities in Seattle’s case, including a $15/hour minimum wage ballot initiative that succeeded in the nearby city of SeaTac in November, and the election of a new Socialist Party Seattle City Council member who campaigned on the issue. But with inequality taking center stage as a political issue in big cities around the country, mayors, businesses, and labor advocates are watching Seattle closely.

However, the focus on big cities shouldn’t obscure the fact that wages are a function of regional economics. Seattle is indeed a big city, with 635,000 residents and (by our count) nearly 500,000 jobs. But it’s only part of King County, Washington, which has roughly 2 million residents and more than 1 million jobs. And King County is just one of three counties that make up the wider Seattle metropolitan area, with a population of 3.5 million and 1.8 million jobs.

While low-wage jobs are prevalent in Seattle, they’re even more prevalent in its nearby suburbs. Using data from the American Community Survey, my colleague Sid Kulkarni and I calculated that between 2009 and 2011, there were on average 149,000 jobs (full-time and part-time) in the city of Seattle that paid less than $15/hour. Over the same period, the remainder of King County had an average of 216,000 jobs that paid hourly wages below that threshold. These low-wage jobs represented 30 percent of all jobs in Seattle, and 34 percent of all jobs in the rest of King County.

It stands to reason that low-wage jobs are more suburban than high-wage jobs. Typically, the highest-value jobs in a region are located in central cities. High-paying sectors like finance, advanced health care, information technology (Redmond notwithstanding), and higher education tend to be more urban than suburban. Yes, cities also have lots of low-paying jobs in hospitality and retail, but so do suburbs. Those jobs tend to follow people, and most people in major metro areas live in suburbs. As my colleague Elizabeth Kneebone has found, as population sprawls, so does low-wage work.

To be sure, many people who live in the King County suburbs of Seattle will benefit from a $15/hour Seattle minimum wage, because they work in the city. According to a University of Washington study conducted for Mayor Murray’s Income Inequality Advisory Committee, fully four in 10 people who earn less than $15/hour working in Seattle jobs—and who would thus presumably benefit from the minimum wage increase—live outside of the city. That’s particularly important in a region like Greater Seattle, where suburbs are home to most of the poor. At the same time, the UW study finds that nearly as many Seattle residents in sub-$15/hour jobs work outside the city limits.

None of this amounts to an argument against Seattle taking the first step toward increasing its minimum wage. Residential and commercial demand is so strong in the city these days that Seattle may have more latitude than its suburbs to boost its minimum wage significantly without encountering negative employment effects. And maybe the city needs to move first in order to convince its neighbors (and itself) that a $15/hour minimum wage won’t make the sky fall.

But these statistics offer an important reminder that the problems of low wages, inequality, and social mobility do not stop at city borders. Ultimately, more cities might try acting in coordination with their surrounding jurisdictions, as the District of Columbia did with two Maryland counties, to boost their minimum wages and ameliorate any “border effects.” And as Seattle contemplates other key policy initiatives, like universal preschool and backfilling state cuts to transit funding (a King County ballot initiative failed last month), it should keep open the lines of communication with its neighbors, and act as one county—or region—where it can.

The Seattle city council voted to push up the city’s minimum wage to $15 an hour. If the wage hike is fully implemented, it will guarantee Seattle’s workers the nation’s highest minimum wage. The increase in the minimum wage will be phased in over a number of years. Big employers that do not provide their employees a health plan are the first that will face the $15 per hour minimum, a requirement that will be fully phased in around 2017. Large employers who offer health benefits will have to pay the $15 minimum starting in 2018. Small businesses with employees who receive tip income will have to pay the $15 per hour minimum a couple of years later, but the countable wage will include employees’ tips. By 2021 all employers in the city must offer a minimum wage of $15 an hour, regardless of the employer’s size.

The federal minimum wage is currently just $7.25 an hour, unchanged since 2009. If Congress does not raise the national minimum wage, Seattle’s minimum will be more than twice the federal minimum wage. Many states currently have a higher minimum wage than the federal one. As it happens, Washington has the nation’s highest state-level minimum wage, $9.32 per hour. Unlike the federal minimum wage law, Washington’s state law increases the state minimum wage every year in line with changes in the consumer price index. By the time Seattle’s $15 per hour minimum becomes effective for large employers in 2017, the Washington state minimum wage will be about $10 per hour, assuming consumer prices continue to rise 2% a year. Thus, large employers in Seattle will have to pay their minimum-wage employees 50% more than minimum-wage employees receive outside the Seattle city limits.

I strongly favor the Administration’s proposal to boost the U.S. minimum wage to $10.10 an hour. It will boost the spendable incomes of millions of poorly paid workers and their families, and I expect it will have only a small adverse effect, if any, on low-wage workers’ job opportunities and work hours. However, I am more cautious about the wisdom of raising a single city’s minimum wage to $15 an hour when nearby jurisdictions leave their minimum wages unchanged.

One reason for my caution is that a big minimum-wage hike can place Seattle’s low-wage employers at a competitive disadvantage compared with employers engaged in the same line of business but located in a nearby suburb. If compensation costs for low-wage workers represent a big percentage of a Seattle employer’s costs, and if the employer faces competition from businesses on the other side of the city limits, companies located in Seattle can lose customers to competitors outside the city.

Consider a business that mainly sells low-cost, fast-food meals. If it must pay $15 an hour to its low-wage employees, while its competitors less than a mile away are only required to pay $10 an hour, the companies outside Seattle can charge lower prices to their customers for shakes, burgers, and fries, and yet still make a profit. The lower cost establishments can capture a larger percentage of the local fast-food trade, reducing fast-food sales inside Seattle’s city limits. The same is true of the goods and services sold by laundry and dry cleaning establishments, inexpensive motels, and other businesses that depend on low-wage workers to stay competitive. The labor cost disadvantage caused by a higher minimum wage can hurt low-wage employment in Seattle and possibly reduce the value of some of the city’s commercial real estate.

To the extent that consumers have the option of buying goods or services from companies that are not required to pay a higher minimum wage, some of the hoped-for gains from a higher minimum wage will be lost. When customers can conveniently buy products or services from firms that face lower labor costs, the new businesses that they patronize will grow and the old, high-cost businesses they abandon will shrink. Low-wage workers may earn higher wages inside the Seattle city limits, but their employment opportunities in Seattle may shrink.

Seattle is a prosperous city, and its mayor was elected in part because of a promise to boost the pay of its most poorly paid residents. If a $15 an hour minimum wage has a chance of working and enjoying broad political support, Seattle is a good place to test the idea. I will be interested to see whether low-wage Seattle businesses continue to prosper even after they are required to pay a minimum wage that is 50% higher than the one faced by competitors in nearby suburbs. The risk of a big minimum-wage hike at the city level is that the city’s low-wage employers will be harmed in their competition with out-of-town businesses that sell the same products or services. The risk of this kind of harm is vastly smaller when the minimum wage is increased at the state or national level. If the Administration can persuade Congress to boost the national minimum wage, all employers—inside and outside a city’s limits—will be required to raise the pay they offer to their most poorly paid workers.

Note: An edited version of this post appears on Fortune.com

Seattle Mayor Ed Murray recently announced a plan to raise the minimum wage in his city to $15/hour over the next few years. The plan emerged from a special business/labor advisory committee, approved by 21 out of 24 its members, after four months of hearings, academic studies, and debate . The measure awaits approval by the City Council, but the move to $15/hour in Seattle seems well underway.

Seattle may be the first, but it won’t be the last, city to take this bold step. Granted, there were some peculiarities in Seattle’s case, including a $15/hour minimum wage ballot initiative that succeeded in the nearby city of SeaTac in November, and the election of a new Socialist Party Seattle City Council member who campaigned on the issue. But with inequality taking center stage as a political issue in big cities around the country, mayors, businesses, and labor advocates are watching Seattle closely.

However, the focus on big cities shouldn’t obscure the fact that wages are a function of regional economics. Seattle is indeed a big city, with 635,000 residents and (by our count) nearly 500,000 jobs. But it’s only part of King County, Washington, which has roughly 2 million residents and more than 1 million jobs. And King County is just one of three counties that make up the wider Seattle metropolitan area, with a population of 3.5 million and 1.8 million jobs.

While low-wage jobs are prevalent in Seattle, they’re even more prevalent in its nearby suburbs. Using data from the American Community Survey, my colleague Sid Kulkarni and I calculated that between 2009 and 2011, there were on average 149,000 jobs (full-time and part-time) in the city of Seattle that paid less than $15/hour. Over the same period, the remainder of King County had an average of 216,000 jobs that paid hourly wages below that threshold. These low-wage jobs represented 30 percent of all jobs in Seattle, and 34 percent of all jobs in the rest of King County.

It stands to reason that low-wage jobs are more suburban than high-wage jobs. Typically, the highest-value jobs in a region are located in central cities. High-paying sectors like finance, advanced health care, information technology (Redmond notwithstanding), and higher education tend to be more urban than suburban. Yes, cities also have lots of low-paying jobs in hospitality and retail, but so do suburbs. Those jobs tend to follow people, and most people in major metro areas live in suburbs. As my colleague Elizabeth Kneebone has found, as population sprawls, so does low-wage work.

To be sure, many people who live in the King County suburbs of Seattle will benefit from a $15/hour Seattle minimum wage, because they work in the city. According to a University of Washington study conducted for Mayor Murray’s Income Inequality Advisory Committee, fully four in 10 people who earn less than $15/hour working in Seattle jobs—and who would thus presumably benefit from the minimum wage increase—live outside of the city. That’s particularly important in a region like Greater Seattle, where suburbs are home to most of the poor. At the same time, the UW study finds that nearly as many Seattle residents in sub-$15/hour jobs work outside the city limits.

None of this amounts to an argument against Seattle taking the first step toward increasing its minimum wage. Residential and commercial demand is so strong in the city these days that Seattle may have more latitude than its suburbs to boost its minimum wage significantly without encountering negative employment effects. And maybe the city needs to move first in order to convince its neighbors (and itself) that a $15/hour minimum wage won’t make the sky fall.

But these statistics offer an important reminder that the problems of low wages, inequality, and social mobility do not stop at city borders. Ultimately, more cities might try acting in coordination with their surrounding jurisdictions, as the District of Columbia did with two Maryland counties, to boost their minimum wages and ameliorate any “border effects.” And as Seattle contemplates other key policy initiatives, like universal preschool and backfilling state cuts to transit funding (a King County ballot initiative failed last month), it should keep open the lines of communication with its neighbors, and act as one county—or region—where it can.