The Lone Banana: A quick look at the opening explorer for white's 7th:Karl A Walbrodt played 7.a3 against no less an opponent than Siegbert Tarrasch in 1898. The game was drawn after 60 moves.
The other two games in the database ( S Henderson vs E Schiller, 1984 and A Madej vs R Bursa, 2001 ) with 7.a3 ended in black wins.
On the other hand, the Opening Explorer lists 7.Bd3 for exactly 4 games, all losses for white. If you like, feel free to claim that the pawn push for denial of b4 is sixteen and two-thirds percent better than the text.
However, clicking on 7.Bd3 brings up a total of 8 games, presumably with the others arriving at the position by a different move order. Of the other four games, 3 were white wins. The one game with the white bishop on d3 when black plays 7. ... Ne4, A S Nordli vs F Remman, 2007, ended 0-1.
The most popular 7th move for white was Nd2. (If this is theory's favorite, would someone explain why?) Results favored white 35% to 24%, with the rest drawn.
Note that the second most frequent white 7th, (the pawn exchange, cd) gave the best results for white for any move that had been tried at least 20 times---45.5% wins, 19.1% losses.
Kibitzing
icon.