Given the choice between shell-on and shell-off pistachios at the grocery store, I always choose the former. The already-shelled pistachios seem like an extravagance reserved for investment bankers and derivatives traders. People with student loans have no business indulging in pre-shelled pistachios, I tell myself. Each nut I split with my bare hands yields not only a tasty treat but also a sense of thrift and virtue.

My beliefs about pistachios are based on assumptions about their price. And we here at Wonkblog tend to be skeptical of assumptions, being believers in the power of empiricism even when it comes to pistachios. So, with the blessing of an indulgent editor, I decided to run the following cost-benefit analysis of shelled vs. unshelled pistachios to test whether my assumptions were correct. What I learned shook me to my salty core.

My materials were an eight-ounce bag of shell-on pistachios and a six-ounce bag of the shell-off variety, purchased from my local grocery store. A quick price-per-ounce analysis seemed to confirm my priors: The shell-on pistachios cost 75 cents per ounce, while the shell-off kind were twice as expensive, at $1.50 per ounce.

But that's not the comparison we want. While the contents of the pricier shell-off bag are 100 percent edible, the shell-on pistachios contain an unknown percentage of shells and empty space. For a proper pistachios-to-pistachios comparison, we need to convert a quantity of shell-on pistachios to its shell-off equivalent.

So I took a cup of shell-on pistachios and set about the tedious work of removing the shells. I was somewhat surprised to find that in the end, it boiled down to half a cup of unshelled product. You need two cups of shell-on pistachios, in other words, to get one cup of edible shell-off nuts.

The two cups of shell-on pistachios in the eight-ounce bag worked out to exactly one cup of ready-to-eat nuts. The six-ounce pre-shelled bag, on the other hand, contained 1½ cups of ready-to-eat nuts. I converted the cost per bag to cost per shelled cup and was shocked to find they were identical: $5.99 per cup of prepared pistachio.

Shell-on or shell-off, in other words, price-wise it makes zero difference when it comes to what ends up in your belly.

But the analysis doesn't end there. Removing shells from pistachios takes time, after all, and as good economically minded empiricists, we here at Wonkblog generally accept Benjamin Franklin's dictum that time is money. So we can't truly compare the costs of shell-on and shell-off pistachios without accounting for the indirect cost of manual shelling time associated with the former.

The simplest way to do this is assume that a person's time is worth whatever their boss pays them to go to work: If you make $10 an hour, an hour of your time is worth $10. So we can multiply a person's hourly wage by the fraction of an hour it takes to shell one cup of pistachios to determine their true cost per ready-to-eat cup when buying the shell-on variety. We don't need to run this extra step for the shell-off pistachios, which require no additional labor on the purchaser's part.

I found it took me almost exactly six minutes, or one-tenth of an hour, to manually remove the shells from one cup of shell-on pistachios at a casual pace. That means it takes 12 minutes of shelling time, or one-fifth of an hour, to transform two cups of shell-on pistachios into one cup of shell-off nuts.

Accounting for labor in this way has drastic effects on the true cost-per-shelled-cup of shell-on pistachios. If you make $20 per hour, for instance, shell-on pistachios work out to nearly $10 per shelled cup: $5.99 for the nuts, plus $4 in labor to remove the shells. If you make $40 per hour, the cost rises to roughly $14 per shelled cup. Recall that the cost of the shell-off pistachios remains the same, regardless of your hourly wage because you don't need to do any work to remove the shells.

In short, we can conclude that, far from being a luxury, buying shell-off pistachios is in fact the fiscally responsible choice. There is no scenario in which the cost savings of a bag of shell-on pistachios justifies additional indirect expenditure in labor.