From No Limit Hold 'em: Theory and Practice by David Sklansky and Ed Miller.
Concept No. 37: Bets on the turn should, on average, constitute a smaller percentage of the pot than flop bets.
This is probably true for the reason given by the authors in their discussion, but they neglect an opposing factor that could conceivably refute this concept's claim.
There are some obvious differences between the flop and the turn. After the turn betting round, there is only one more card to come and one more betting round. After the flop, there are two more cards and two more betting rounds. As Sklansky and Miller discuss, both of these factors favor the EV of draws on the flop over the turn. It follows that, in order to make it unprofitable for a drawing hand to call, a made hand would need to bet more on the flop than on the turn.
However, there is another obvious difference between the flop and the turn that can have an effect of the EV of draws, and this one favors the EV of the draw on the turn. I am referring to the fact that while there are only three cards on the board after the flop, there are four after the turn. This extra card means that the board has much more potential to threaten multiple draws, which means that it will be less obvious which draw your opponent has. This, in turn, means that each draw has higher implied odds, because (as Sklansky and Miller pointed out in Concept 31) if your opponent hits his draw, it will be very difficult for you to figure it out. To combat this, you will often have to bet extra on the turn if you have a made hand. This factor is completely ignored by S+M.
So, on the flop, draws have the benefit of an extra round of betting to extract value if they hit on the turn, plus the a possibility of getting two tries to hit the card (although they may have to call another bet on the turn in order to see the river). These factors increase the EV of draws on the flop, and thus demand bigger bets from made hands.
On the turn, draws have the benefit of some extra "cover" because there will often be more draws on the board than on the flop. These factors increase the EV of draws on the turn, and thus demand bigger bets from made hands.
Although I suspect the former factor is more significant, which would make this concept's claim correct, it's not entirely clear. What is clear is that the latter factor was ignored by the authors in the book's discussion.