Will Wilkinson points out a recent studying the relationship between political ideology and his five foundations theory of moral sensibility and judgment. These 5 foundations are:

1) Harm/care, 2) Fairness/reciprocity, 3) Ingroup/loyalty, 4) Authority/respect,5) Purity/sanctity more elaboration here.

Apparently, the survey was more intended to primarily focus on liberal-conservative differences, but a large number of self-identified libertarians(1/8 of the sample population) took the survey, which allowed the authors to draw conclusions about libertarians as well as conservatives and liberals.

"Liberals care most about the Harm/care and Fairness/reciprocity foundations"

"Conservative concern was spread more evenly over the five foundations, and they were less willing than liberals to violate Ingroup, Authority, and Purity for money."

Libertarians, on the other hand, had the lowest sacredness scores on all five foundations. Jonathan Haidt, the author, concluded: "Free-market libertarians appear to be the least outraged and most open to contractualizing moral violations. The differences were particularly stark between libertarians and conservatives on the three binding foundations. Libertarians may support the Republican Party for economic reasons, but in their moral foundations profile we found they more closely resemble liberals than conservatives."

This led Wilkinson to write: "Libertarians are liberals who love markets."

From a political philosophical position, libertarianism, at least historically, is a radical branch of liberalism that replaces the so-called social contract with a market contract(s).

Haidt responded in the comments that the Editors of his original paper cut at quite a bit of his libertarian comparison study observations, so he is working a new paper comparing libertarians to liberals and conservatives on a number of scales. He gives a succinct summary of what he is finding:

Libertarians are liberals who lack bleeding hearts. Libertarians look much more like liberals than like conservatives on most measures, EXCEPT those that have anything to do with compassion, on which libertarians are lower than liberals AND conservatives. The lower levels of compassion, and higher levels of need for cognition and tendency to "systemize" rather than empathize, are probably related to the love of markets.

I'm not sure about the lack of empathy part, at least from my experience, but lack of "sacredness" is pretty spot on. Just because I don't not respond with "there outta be a law" to every social event I may not agree with doesn't mean I lack empathy; the ability to apply a systematic mode of thought to foresee the unintended consequences, and thus conclude why there often "shouldn't be a law" is not an absence of compassion, at least in my book. And from Herbert Gintis' research, for example, we can observe the key role markets play in facilitating empathy among members of heterogeneous groups.

But from Haidt's research, perhaps we can dispense with the nonsense of the oft quoted refrain that "libertarians are conservatives who like to smoke pot." A more accurate quip would be that "libertarians are liberals mugged by economic and political reality."