Among other disappointments for liberals last Tuesday was the failure of California’s Proposition 19, which would have rewritten state law to allow local jurisdictions the right to regulate and tax the use of marijuana for personal consumption.

The measure, which was defeated 54 to 46 percent, had seemed destined to lose after polls found its position slipping in the final few months of the campaign.

Still, the defeat was a bitter one for advocates of liberalized drug laws, particularly since liberals had a strong night in California over all, re-electing Democratic Senator Barbara Boxer, electing a new Democratic governor, Jerry Brown, and soundly defeating another ballot measure, Proposition 23, which would have suspended California’s stringent air pollution laws until its unemployment rate declined.

Proponents of marijuana legalization, like the group Norml, have put a happy face on the measure’s defeat, nothing that the 46 percent of the vote it achieved is better than any similar initiative in any other state, and that national polls show support for legalization having increased significantly over the past 10 or 15 years.

Others have been more skeptical, however. Tyler Cowen, a libertarian-leaning economist at George Mason University who writes columns for The Times, commented on his blog that “we’re seeing the high water mark for pot, as aging demographics do not favor the idea,” and that he couldn’t see marijuana “climbing the legalization hill, if it can’t make it through current-day California.”

The Atlantic’s Megan McArdle expressed similar sentiments, noting that parenthood — and the changes in attitude it can cause toward drug legalization — was a significant barrier to such initiatives passing.

The relationships between age, parenthood and views on marijuana are a bit complex, so it’s worth going to some effort to untangle them.

The chart that follows contains data from the General Social Survey, a broad-themed poll conducted by biannually by the National Opinion Research Center at the University of Chicago, which for the past 38 years has asked adults whether they think marijuana should be legalized. I have combined data from the 2000, 2002, 2004, 2006 and 2008 editions of the survey to improve the sample size — about 6,000 respondents are represented in all. (The data presented here are unweighted based on demographic characteristics, and the responses from people who said they were unsure about marijuana legalization are omitted.)

Positions on marijuana legalization are broken down based on two categories: first, the age of the respondent, and second, their parental status. We distinguish between those adults who currently have a child under the age of 18 living with them, and those who fathered or mothered a child at some point in their lives, but no longer have one living with them.

As you might expect, both age and parenthood status are important predictors of views on marijuana. Controlling for parenthood status, support for legalization is actually quite steady from ages 18 through 59: about half of childless adults in these age ranges favor legalizing marijuana.

The percentages are about 10-15 points lower, however, for adults in this age range who have had kids. It is not clear whether it makes much difference if the child is still living in the household or not: controlling for age, support appears to go up a couple of points among “empty-nesters” (relative to parents who still have children in their homes), but the difference is not clear enough to qualify as statistically significant.

Among adults aged 60 or over, however, support drops quite precipitously. The gap between childless and child-bearing adults also diminishes, and vanishes entirely for adults aged 70 and up: few septuagenarians, no matter how many children they have or haven’t had, want any part of liberalized drug laws.

The sharp break among adults in this age range is probably not a coincidence; it has a very strong relationship with lifetime usage rates for marijuana — and reflects, in essence, the divide between those adults that came of age before and after the counterculture of the 1960s and the Summer of Love.

This, then, is the good news for supporters of marijuana legalization. Even if nobody was persuaded about legalization one way or another, measures like Proposition 19 would stand to gain ground as pre-Baby Boom adults were replaced in the electorate by Millennials.

Although the chart above does not reflect it — perhaps because it relies on some data that is as much as 10 years old — there is some evidence that Millennials are coming into young adulthood with somewhat more liberal views about drug use than their predecessors in Generation X, who may remember the “Just Say No” years and the crack cocaine epidemic of the early 1990s. Exit polls in California found a particularly sharp break between adults aged 18 to 24, 64 percent of whom supported Proposition 19, and those aged 25-29, 52 percent of whom did.

Even if Millennials had about the same attitudes toward drug legalization as Generation X-ers did, the pro-legalization position would still stand to gain some ground simply because both groups are much more tolerant of drug use than adults over the age of 60.

It is also the case, however, that support for legalization drops sharply once adults have become mothers or fathers. These effects can lead one to exaggerate the extent to which support for marijuana legalization is tied to age. Since adults in their 40s will be more likely to have had children at some point in their lives than those in their 20s, they will appear to have a more anti-legalization position over all. Controlling for parenthood status, support for legal marijuana is about the same among these age groups — and we can probably expect that some of the 20-somethings who support legalization now will no longer do so once they have children of their own.

Suppose we are in a world in which: (i) members of the pre-Baby Boom generation are eventually replaced in the electorate by Millennials, who have relatively liberal views on marijuana; and (ii) nobody else ever changes their views on marijuana, other than if they have children, when some of them go from being in favor of legalization to against it.

In this world — which is a reasonable model of America in 2010 — support for marijuana initiatives would continue to increase for a few years until adults now over the age of 60 were no longer a significant part of the population. Then it would enter some sort of steady state.

I suspect this steady-state condition is one that would support some measures like Proposition 19 passing in the right states (e.g. the more liberal ones) at the right times. Particularly if the measures were well written: Proposition 19 lost standing in the polls as the election wore on, in part because its opponents focused on the fact that it would create a patchwork of local laws and an additional layer of bureaucracy; a simpler measure might have passed or come closer to it.

I somewhat doubt, however, that this world is one that would imply an overall majority of Americans being willing to support legalization — my back-of-the-envelope calculation suggests that support would tap out at something on the order of 45-47 percent nationally. Of course, 45 percent support nationwide would probably translate to better than 50 percent support in states like California, Oregon, Nevada, New York, Vermont and so forth. But it probably would not lead to a world in which, for instance, the Democratic nominee for President were willing to support marijuana legalization as part of his platform (and certainly not the Republican one).

I also suspect that national polls would have to show somewhat better than 50 percent support for legalization — perhaps 55 or 60 percent — before changing federal drug laws become a routine part of the national conversation; there are vanishingly few prominent politicians who have been willing to embrace marijuana legalization, including those who take liberal positions otherwise.

I also don’t know that marijuana legalization can be thought to be on quite the same trajectory as something like, for instance, gay marriage — an issue which also is befitting from generational turnover, but one on which the intra-generational movement seems to be more uniformly in the direction of tolerance. One can also look toward Western Europe for guidance, for instance, which liberals think of as having more enlightened views on social policy. Seven European countries now provide for gay marriage, and 13 others for registered domestic partnernships, and those numbers will probably continue to increase. But, although many European countries take a laxer attitude toward marijuana than the United States does, none have legalized it — including the Netherlands, which has merely decriminalized it. And even Holland has been backtracking some: the number of “coffeeshops” — authorized marijuana retailers — has declined as new restrictions have been placed on opening such and as some local communities have become stingier about handing out licenses; the Netherlands also recently banned the sale of psychedelic mushrooms, which it used to tolerate.

That is not to say that advocates for marijuana reform have a hopeless task ahead of them. The uptick in support for marijuana legalization over the past decade or so is probably greater than can be explained by generational turnover alone, and the country seems to be going through a libertarian phase of sorts. My hunch is that the key “swing” demographic could turn out to be empty-nesters: adults whose children are grown. If their positions on marijuana legalization were to become more like those of adults who had never had children in the first place, that would probably be enough to give the pro-legalization side a majority — but likely not otherwise, particularly since the share of “empty nesters” is increasing as a share of the population as adults live to a longer age.

Finally, Proposition 19 suggests that marijuana legalization initiatives may lose some support at the ballot booth relative to where they appear to stand in polls initially. Part of this is because — even in years in which Democratic turnout is comparatively strong — younger adults are considerably less likely to register and to vote. But part of it may also be that the anti-legalization argument has some intrinsic advantages that are liable to become manifest over the course of an actual campaign, such as its appeal to parents.