There is a considerable body of research, most of it collected through studying twins and adopted siblings, which says parents don't matter to the degree that moms and dads and the piano teacher industrial complex say they do. This research says that nature (genes) tends to outweigh nurture (parenting) when it comes to such things as character, health, and chances for success.

Svisio/iStock/Getty Images
Of course, your kids usually get your genes and your parenting, so you're to blame either way.

Does this mean parents don't make a difference? No one is claiming that. The data says that parents still have the power to steer kids to smarter choices or to screw them up, particularly in the short term. The question is: How big of a difference do parents make in the long term? Parental influence is still being studied, and new information is surfaced every year, but the weight of the evidence currently bends in the direction of nature playing a significant role, particularly when it comes to ...

Just as with money, there is evidence that genes have an effect on grades. In 2013, British researchers studied the academic performance of more than 11,000 identical and non-identical 16-year-old twins. They found that genes contributed more to students' grades than teachers, schools, or family environments. These findings have been repeated in other studies, as well as on the 1980s television program Diff'rent Strokes, where Willis sometimes struggled with his grades even though he and his brother Arnold had been adopted by a wealthy Manhattan industrialist.

An analysis of Korean-American adoptees found that being raised by a college-educated mother increased an adoptee's probability of graduating from college by 7 percent. By comparison, a child with a college-educated biological mother had a 26 percent increase in the probability of graduating from college. It didn't matter if the adoptive parents were rich or educated.

christingasner/iStock/Getty Images
Or even if the kids wrote poignant essays about their pride in being adopted.

Perhaps, looking back on it, this is what Willis was talking about.

In the book Selfish Reasons to Have More Kids, economist Bryan Caplan says that parents grossly overestimate how much they can actively influence a child's happiness. He bases this conclusion on data showing that identical twins (who share identical DNA) are far more similar in their happiness levels than fraternal twins (who do not share the same DNA).

Thinkstock/Stockbyte/Getty Images
Of course, identical twins are naturally happier because they indulge in wacky twin hijinks.

Good news for parents of sullen teens: Research shows that unhappiness is largely genetic, rather than the result of external factors, like whether your mom hugged you enough. "By the time you're an adult," Caplan writes, "your parents' past mistakes are not the reason for your present unhappiness."

To which he should have added, "So, good luck with that."

Research on twins (are twins only bred for science now?) says that genes do influence personality, but it's not known yet how. Billions of dollars of research into human DNA has yet to turn up specific genes for determining things like doing your roommate's dishes for them or politely waving to the car behind you when its driver lets you merge into traffic. (Please keep looking, science, we need more of these people.)

Can biology explain criminal behavior? More than 100 studies show that inherited traits play a role in crime. But just because someone is predisposed to criminal behavior does not mean they will become a criminal. Nurture can play a role in subduing criminal impulses and redirecting them to more productive pastimes such as MMA or serving in Congress.

Image Source Pink/Image Source/Getty Images
"You're right ... being able to rob the whole country at once is so much more efficient! Thanks!"

Whether a person is fat as a child or as an adult is a trait that is "substantially heritable," according to researchers.

Scientists have even isolated a gene in mice that affects weight (so if you start to see more sexy slim mice, this is probably why). The mice in the study ate the same amount of calories as their peers, but with one genetic alteration they gained weight. Scientists hope this information will help humans someday with weight gain and loss.

David De Lossy/Photodisc/Getty Images
Dietitians recommend eating fewer than five mice per week.

This is because it is known that human genes can influence how much food turns into fat and how much food gets burned off. The good news is that diet and exercise can still keep one fit. The bad news is that being skinny is way harder for those of us who still hide broccoli in the furniture.