Those who deny that germs cause disease shouldn't call their opponents anti-science. But that's exactly what HBO comedian and germ-theory-denier Bill Maher routinely calls Republicans to hearty applause.

The core trait of a scientific mind is that when its commitments clash with evidence, evidence rules. On that count, what grade do liberals deserve? Fail, given their reaction to the latest evidence on universal health care, global warming, and universal preschool.

The policy world was rocked recently by a New England Journal of Medicine study showing that Medicaid doesn't improve the health care outcomes of uninsured individuals.

The study compared the health status of adults who were randomly enrolled in Oregon's Medicaid program with those who weren't. It found that two years after patients received Medicaid, "no significant improvements in measured physical health outcomes" such as hypertension, cholesterol and diabetes resulted. Coverage did, however, lower depression rates and reduced financial strain.

How should a scientifically-inclined liberal have reacted? By acknowledging that if the findings hold in subsequent years, Obamacare's plan to use Medicaid to achieve its universal coverage goal—at half-a-trillion-dollar price tag over a decade—would need to be reconsidered.

Some liberals such as Ray Fisman of Slate did just that—but they were the exception. Most liberals either dissed the study's methodology after praising it previously (Kevin Drum, Mother Jones) or ignored its core findings and reported the good news (Jonathan Cohn, The New Republic) or attacked Obamacare's opponents as heartless fools (Paul Krugman of The New York Times).

For two decades, progressives have castigated those questioning global warming as "deniers."

But the Economist, once firmly in the alarmist camp, recently acknowledged that global temperatures have remained stagnant for 15 years even as greenhouse-gas emissions have soared.

This may be because existing models have overestimated the planet's sensitivity. Or because the heat generated is sinking to the ocean bottom. Or because of something else completely.

How should a scientifically inclined liberal react to this trend? By inhaling deeply and backing off on economy-busting mitigation measures till science offers clearer answers.

And how have liberals reacted? By sticking their fingers in their ears and chanting la-la-la.

The New York Times editorialized this week that the European Union should redouble its efforts to salvage its floundering carbon trading system. This scheme forces companies that exceed their greenhouse gas emission quota to either reduce production or spend gobs to buy spare quotas from others.

The Washington Post's Brad Plumer penned an essay noting that atmospheric carbon emissions are now approaching levels only seen in the Pliocene Era—without bothering to note that they aren't producing the same warming this time. Most priceless, however, was Chris Mooney, author of The Republican War on Science.

He spilled serious ink in Mother Jones defending the highly questionable "hockey stick" graph—the core evidence of global warmists—which allegedly showed a sudden warming spike in the last century after a millennium of steady temperatures.

Liberals don't just want universal health insurance—they also want universal preschool. But the evidence for government-funded preschool is even more dubious than for government-funded health care.

Numerous studies on Head Start, the federal pre-K program for poor kids, show that its reading and math gains virtually evaporate by fourth grade. And the latest evidence from Oklahoma and Georgia, two states that implemented universal pre-K in the 1990s, only confirms this.

Oklahoma's high-school graduation rates have dropped since it embraced UPK and Georgia's remain stagnant. The average reading score of Oklahoma's fourth graders on the NAEP—the national report card—dropped four points between 1998 and 2011.

Georgia just reached the national average. The NAEP reading gap between black and white children in Oklahoma was 22 points in 1992. In 2011? The same. Georgia had a 28-point spread in 1992. In 2011? Twenty-three points.

How should President Barack Obama, who had promised evidence-based policy, have responded? By renouncing his commitment to UPK. What did he do? Jetted to Georgia and declared its program a national model.

It's not that conservatives don't have ideological fixations that are impervious to science. However, they don't pretend to don the mantle of science. Liberals do.

This column originally appeared in the Washington Examiner.