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In addition to the problem of mutation accumulation by relaxation of selection, when a population has begun shrinking, as is the case for the native populations of all Western and developed nations, there is an increasing danger of extinction due to 'mutational meltdown' - when deleterious mutations accumulate so rapidly that they overwhelm a population before it can evolve an escape.
(http://en.wikipedia.org/wiki/Mutational_meltdown) .
Mutational meltdown was first described as a threat for small populations of asexual organisms; later this was widened to sexual organisms and then to large populations - so mutational meltdown has gone from being a specific case to probably a universal possibility.
The unusual twist with modern humans is that populations have begun falling due to chosen sub-replacement fertility, and before mutation accumulation has reached a level sufficient biologically to suppress fertility. In other words psychological factors have anticipated biological factors - and presumably both psychological and biological population decline will combine to increase the degree of reduced fitness resulting from mutation accumulation.
This will probably have increased the risk of mutational meltdown, and of extinction.
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