by traditional definitions, the percentage of children living in “single-parent” households has risen steadily over the past few decades, both nationally and in Massachusetts. One measure is the share of all births to unmarried mothers; nationally, that’s gone from 28 percent in 1990 to 34 percent in 2000 to an estimated 40 percent in 2007. In the Bay State, the corresponding figures are 25 percent, 27 percent, and 33 percent. (Mississippi has hit 54 percent, and Utah is at the other end of the scale, with 20 percent.)

This trend is worrisome, given the numerous studies pointing to a correlation between single parents and poverty, but the numbers are skewed by a growing demographic group in America: children living with unmarried parents. Until recently, the Census Bureau counted any child without a legally married mother and father as living in a “single-parent” home. But the 2006-08 American Community Survey added estimates on the number of children in homes with an “unmarried partner of householder present” — mostly referring to opposite-sex couples but also including all same-sex couples in Massachusetts, since the federal government does not recognize any of them as married. The map below shows the percentage of children in larger cities living in homes without coupled adults, married or otherwise. The 2010 Census will be the first to include this data for all communities.

In Northampton, the percentage of “single-parent” children drops from 40 percent to 30 percent under the new definition. In New Bedford, 58 percent of children live in households without married parents, but the number drops to 46 percent (still an alarming number, to be sure) when unmarried partners are counted. Even under the new definition, a majority of children now live in households with one parent in Lawrence, Holyoke, and Springfield. And in addition to New Bedford, the figure is above 40 percent in Boston, Fall River, Lynn, and Worcester. But the redefinition pushes the number down to a mere 10 percent in the exurbs of Billerica and Plymouth.