The company last included Stern in its survey in 2009 but says his numbers tend to be pretty stable.

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Keefe points out that government jobs tend to be more stable, and this almost certainly explains a large part of the gap.

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Investors are attracted to many of the biggest companies, the argument goes, because such firms generate much of their earnings growth overseas and tend to be in more stable industries.

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"Markets with a high percentage of jobs in those categories tend to be more or less stable markets, " says LMM president Ingo Wizner.

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These places also tend to be inhabited by reasonably well-educated people whose stable incomes makes them less vulnerable to contractions in competitive industries like finance, manufacturing, construction or information.

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In particular, as William Amos, one of Ms Acevedo-Whitehouse's colleagues, points out, levels of inbreeding tend to be lower in expanding populations than in stable ones.

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Its properties tend to be leased to either government agencies, government contractors like Lockheed Martin and General Dynamics, or stable businesses.

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