What if I said, nowhere? Now, we're going to keep this discussion to the United States only, in case you are itching to trot out statistics from Kazakhstan or Sweden. Let's start with Audi's ridiculous Super Bowl ad.

If you don't want to watch, the text begins with a dad (apparently a dad who never once read a parenting book or listened to his own parents) who says, "What do I tell my daughter?" He then goes on to say all these horrible things about how she'll be treated poorly because of her gender. "Do I tell her that her grandpa is worth more than her grandma?" and "Do I tell her that despite her education, her skills, her intelligence, she will automatically be valued as less than every man she ever meets?"

Goodness, no, dad. What kind of a parent sets out to tell his child that she'll be an utter failure? Oh wait, that's not the point. The point is the pay gap.

Do women, as a group, earn less than men as a group? Yes. Does that mean there is a gender biased pay gap? No, not at all.

Does Even Spiegel earn twice as much as Bobby Murphy? Yes. Does that mean that there is some sort of bias against Murphy? No, not at all.

Pay isn't something magical from on high. It's based on numerous factors, all of which boil down to market rates. Companies don't want to pay you any more than they have to--whether you're male or female--and you don't want to earn less than you're worth. Of course, this Audi dad is priming his daughter to think that even with a Harvard MBA she'll be lucky to make minimum wage, but I digress.

So, why do men, overall, earn more than women, overall? One word: Choice.

Women prefer to not do jobs that are dangerous. In 2013, 3,635 men died in workplace accidents, compared to 950 women. Men are far more willing to take on dangerous tasks, and dangerous jobs pay more than safe jobs.

Women prefer to work fewer hours than men do. The statistics use the term "full-time" to describe anyone over 35 hours a week but doesn't compare apples to apples. The Bureau of Labor Statistics notes: "even among full-time workers (those usually working 35 hours or more per week), men worked longer than women--8.2 hours compared with 7.8 hours." That may not seem like much, but that means 41 hours per week for full-time men, to 39 for full-time women. That's more than 100 hours per year difference. That's a big difference.

Women often have caregiver obligations. You can say that it's societal pressure, and therefore, unfair, but you choose with whom you reproduce. There's no law that requires mom to be the primary caregiver. There's also no law that says daughters have to take care of aging parents rather than sons. It's all about choice. Anne-Marie Slaughter, in a book called Unfinished Business notes:

If you take women who don't have caregiving obligations, they're almost equal with men. It's somewhere in the 95 percent range. But when women then have children, or again are caring for their own parents or other sick family members who need care, then they need to work differently. They need to work flexibly, and often go part-time.

Women prefer temporal flexibility. Harvard Economics Professor Claudia Goldman looked at people with professional level jobs and found that woman strongly preferred flexibility over higher pay. She said, in an episode of Freakonomics:

By and large, it appears that there's just a very high cost of temporal flexibility in certain occupations. And part of this is that people don't have good substitutes for themselves in certain cases. So, you are doing a merger in an acquisition, you're a lawyer, you are a consultant -- whatever it is -- the client might say, "I want you there. I want you there all the time. I want to call you at 2 in the morning. I want you to be there on Sunday, on holidays. I want you to go to Japan whenever I say that you should." Well, that's a tremendous demand. So an individual who values their family time would say, "I'm not doing that." So therefore if a woman wants to -- law, for example, is a good example -- if a woman wants to practice law, she has a law degree, she enjoys practicing law, being a corporate counsel would give her more flexibility. That doesn't mean that she's working fewer hours than she would've worked otherwise, but she can work her hours, and she gets paid somewhat less.

So, what does all this have to do with the Snapchat co-founders pay discrepancy? Well, unlike most people who don't have access or information about their co-workers' salaries, as a co-founder, Murphy probably undoubtedly knew what his co-founder earned. Which probably means that Murphy and Spiegel had different responsibilities and different priorities. That's okay too. There's no need for everyone to make the exact same amount of money, even if they have similar titles.