It's a theory I accept without proof.

But it's nice to have proof all the same.

Look around the League, and show me a team with great defense, and I'll show you a team with great veterans.

I don't know precisely why that is -- I suspect part of that has to do with "earning the respect of the referees."

But today, I think for the first time, there is some proof.

Kevin Pelton (of Basketball Prospectus fame) pored through nearly a decade of 30 teams' data, and has come up with some nice evidence showing a trend -- team defensive efficiency improves with age. There is a big and easy-to-read explanation at BlazersEdge.

How big is the effect? Pelton estimates about 19% of a team's defensive prowess is explainable simply by their age. Well worth reading the whole thing.

And one other finding that fell out of the analysis and is well worth noting: Remember the exciting young Bulls teams of 2004,-2005 and 2005-2006? That was a very rare case of a young team that excelled at the less sexy end of the floor.

What drives this? Who knows? Maybe everyone arrives in the league expecting to make highlight reel offensive moves, while older player realize they want to win, and focus on boring things like D. Maybe players who don't defend well don't last very long in the NBA, so there's improvement with age by natural selection, if you will. Maybe some elements of defense are secrets that take years to learn. Maybe veterans make so much money that it's worth doing just about anything to impress coaches, be irreplaceable, and extend their careers.

One thing that might be something of a factor is that older teams get called for slightly fewer fouls. Justin Kubatko of Basketball-Reference was nice enough to quickly cobble together the graph to the right in response to my question about that issue.

On the bottom axis is a team's age, as determined by minutes played (this way it's not skewed by the injured or those who seldom play).

On the vertical axis are fouls per 48 minutes.

The points on the graph represent every NBA team from 2001-02 through 2007-08.

You can see that with age, there is a slight trend toward fewer fouls.

"The correlation between weighted team age and fouls per 48 minutes,"

explains Kubatko, "is -0.32. So there is a negative association

between the two variables, but it's not particularly strong. An

increase of one year in weighted team age would lead to a predicted

decrease of 0.3 fouls per 48 minutes. (By the way, it's just a

coincidence that the correlation coefficient and the slope of the

regression line are roughly the same.)"

Now, why do veteran teams get called for fewer fouls? Is there some referee bias against youngsters? Are veterans just better at not breaking the rules?

I don't think we have evidence to answer that question with any kind of certainty, but it's a good topic for another day.

In the meantime, what I take away from this is some support for the long-held theory that young players who stick in the NBA will generally tend to become more effective defenders with time.

UPDATE: TrueHoop reader Brian has an interesting question:

Another potential factor is that older teams might simply play at a slower pace. I noticed the graph that Justin provided was for fouls per 48 minutes; I wonder what happens if the graph is adjusted to provide fouls per 100 possessions -- would the correlation go away, or be softened? It might not, but it's something to look at.