We all want to succeed in life, and when we become parents, we want to do everything we can to make sure our kids are successful, too. Now a new study says there's a single decision many parents make that can effect whether their kids have an advantage or a disadvantage for much of their entire academic career.

The question is a simple one, and it's well-known to any mom or dad whose child has a late birthday: Do you enroll your child in school as soon as he or she is eligible, or do you wait a year?

If you read no further, know this: For all the complexity and controversy in this issue, researchers at Stanford University say that kids whose parents hold them back a year have significant advantages over their peers.

In other words, in the "red-shirting versus not red-shirting" battle," red-shirting wins, hands down. Here's why.

Out of control--a 'persistent' result

Many parents make this decision based on the whether their kids will benefit academically by waiting a year or not. But the science on testable academic performance yields frustrating, inconsistent results.

That's why Stanford researchers decided to set their sights on something different. Cast aside the test scores; how did being among the oldest kids or the youngest kids in the class affect things like mental health, discipline, and self-control? (All of which can ultimately have a greater effect on qualitative academic achievement.)

To assess the effect, which was reported recently in Quartz, they studied the experiences of elementary school students in Denmark, segmenting them into groups whose parents had enrolled them during the school year in which they were first eligible, and those whose parents decided to hold them back for a year.

Result? Kids who delayed attending kindergarten to the later year were far more likely to be able to pay attention in school and had "dramatically higher levels of self-control" than their peers. And that advantage was sustained for years afterward.

"We found that delaying kindergarten for one year reduced inattention and hyperactivity by 73 percent for an average child at age 11," Thomas Dee, one of the co-authors, said. "And it virtually eliminated the probability that an average child at that age would have an 'abnormal,' or higher-than-normal rating for the inattentive-hyperactive behavioral measure."

The 'relative age effect'

Even if you don't have kids, and even if you don't remember your elementary school years particularly well, this whole debate might ring a bell. It was discussed thoroughly in the media a decade ago, after Malcolm Gladwell came out with his book, Outliers.

Malcolm studied the National Hockey League (he's Canadian; go figure), and noticed a statistical anomaly about the number of players who were born in January and February. He traced it back to the fact that in Canada, youth hockey leagues most often used January 1 as the birthday cutoff date.

That meant that kids who were born in January and February were always the oldest kids in their age-group. Sure, that meant they were more mature and physically developed, but there was another advantage. Their advanced physiology led to more playing time and coaching attention. That in turn, led to more success on the ice.

The phenomenon, called the relative age effect, also occurs in academia. Previous studies have suggested that children with a late start, and whose birthdays were earlier in the year than their classmates, were more likely to attend college, and less likely to be put on a vocational track (as opposed to an academic track) in school.

So, should you red-shirt your kids?

In the United States, about 20 percent of kindergartners are red-shirted, meaning their parents decided to hold them back before entering school, so they experience kindergarten as six-year-olds, rather than five-year olds.

All things being equal, it seems this gives them an advantage. But it's worth looking at some of the reasons why "all things might not be equal."

First, wealthier families are much more likely to red-shirt their children. (Some parents who might want to consider red-shirting might find it prohibitive, because not going to public kindergarten means going to daycare.) But this leads to a question whether wealthier red-shirted children might be benefiting in the self-control department from some other aspect of their more privileged childhoods.

Second, and related, there's the question of what kids do during their "extra year." In Denmark, the researchers said, there is universal access to decent pre-kindergarten. Relatively few American cities and towns offer the same thing.

Still, if your kids are on the edge it seems red-shirting brings undeniable advantages. The difference in development between age five and age eight is significant, and as the study notes, even if they spend an extra year soaking up "an extended (and appropriately timed) exposure to such playful environments" might have a big advantage.