With the sudden resurgence in the movement to legalize same-sex marriage over the last few weeks, it occurred to me that the addition of a few more large states to the Marriage Equality Club would increase the number of Americans eligible for gay marriage to nationally significant levels. Then it dawned on me, I actually had no idea what proportion of Americans are living in states with legal gay marriage. That is to ask, what percentage of the U.S. population is comprised by the jurisdictions that have legalized same-sex marriage (SSM)? How has this number changed over time? I’ve never seen the figure in any news source. Neither Google nor Wikipedia prove helpful. So I decided to gather the raw data myself and tackle the problem.

Using data from the 2010 census and annual census estimates from 2004 to 2011, I’ve built a reasonably thorough and accurate picture of the share of Americans living in jurisdictions where SSM is legal and how that figure has changed over the last eight years. Here’s what I found:

11.32% of the U.S. population currently lives in a jurisdiction in which same-sex marriage is legal.

The graph below shows how this figure has changed since gay marriage was first legalized in Massachusetts on May 17, 2004. For clarity, the y axis runs from zero to 20%.

This second graph shows the same data on a scale from zero to 100%, for the sake of eliminating graphical distortion. The yellow line highlights what the value would be (30.30%) if the four states, Washington,* Maryland,* New Jersey and California legalized same-sex marriage on the day of this post.

Major Findings

Though not easily visible on the graphs, the horizontal line segments actually have a slight, negative slope. This is due to the fact that states favorable to the legalization of gay marriage have tended to be those states steadily losing their share of the U.S. population. For example, when Massachusetts legalized gay marriage in 2004, it held 2.20% of the total U.S. population. By 2011, that figure had dropped to 2.11% and its decline shows no sign of abatement.

We might call 2009 the Golden Age of Gay Marriage. The greatest concentration of SSM legalizations occurred in a 14 month period immediately following the passage of Prop. 8 in California. Five of the seven jurisdictions which have permanently legalized same-sex marriage did so in this period. Merely a week after Prop. 8’s passage, Connecticut kicked off a spate of legalizations which lasted until New Year’s Day, 2010. The share of Americans living where SSM is legal more than doubled from what it had been after Election Day 2008, when Prop. 8 left Massachusetts, once again, the lone state in the club. I find myself wondering if these legalizations were part of some broad reaction within the bluest states against the ethos exposed by California’s Prop. 8.

The Golden Age of Gay Marriage ended with legalization in New Hampshire, which was followed by a year and a half of nothing. The plot below shows a close-up of the Golden Age.

The legalization of SSM in New York doubled the population again, getting within striking distance of the record high set by California (14.17%) during its brief foray into marriage equality in the summer of 2008. The addition of any two probable candidate states, such as Washington* and Maryland,* would send the population share of the Marriage Equality Club to an all time high. The return of California would more than double it. If the four states, Washington,* Maryland,* New Jersey and California, legalize gay marriage, Americans living where gay marriage is legal will exceed 30%, or nearly one in three. Gay marriage would no longer be a fringe, regional oddity limited to the bluest of the blue states; it would prove its status as a nationally bifurcating issue, like abortion.

The Data

The table below shows the data points I calculated to build the graphs above. The two columns on the right show the percentage of the U.S. population living in jurisdictions where same-sex marriage is legal, both before and after each legal event. Note that due to demographic changes, these percentages do not remain constant over the periods of inactivity.

Date Event Before After 2004 May 17 Massachusetts 0% 2.20% 2008 June 16 California 2.15% 14.17% 2008 November 5 California Prop. 8 14.17% 2.15% 2008 November 12 Connecticut 2.15% 3.30% 2009 April 3 Iowa 3.29% 4.27% 2009 September 1 Vermont 4.27% 4.48% 2009 December 18 District of Columbia 4.47% 4.66% 2010 January 1 New Hampshire 4.66% 5.09% 2011 July 24 New York 5.07% 11.32% with Washington* alone 11.32% 13.51% with Maryland* alone 11.32% 13.19% with New Jersey alone 11.32% 14.15% with California alone 11.32% 23.41% with all four 11.32% 30.30%

I will post an update when the next state legalizes same-sex marriage.*

Sources of Raw Data

*As of this post, the governments of Washington and Maryland have passed legislation legalizing same-sex marriage, but it likely that the opposition in both states will successfully block these laws from taking effect, at least for the time being.