The horses with narrower IMWs tended to have narrower bodies, and performed better in distance races, which account for a higher percentage of graded stakes events than shorter races. The horses with wider IMWs tended to have wider bodies and performed better at short distances. Consequently, IMW seemed to be a better indicator of distance potential than racing class.
Many horsemen believe that a wider jaw allows for greater air exchange, which results in better performances by horses on the racetrack. There also have been reports in scientific literature that support the wider-is-better hypothesis, thus popularizing the theory in the equine industry involving intermandibular width (IMW). However, a recent study by Dr. P.S. Mostert and Dr. D.D. Householder showed that horses with wide jaws are not necessarily more successful athletes than their narrow-jawed counterparts. The results of the researchers' work appeared last year in an edition of The Journal of Equine Veterinary Science. After measuring the jaws and looking at the racing performances of more than 2,000 Thoroughbreds, Mostert and Householder concluded that horses with greater IMWs did not race at higher levels or at longer distances compared to horses with lesser IMWs. The researchers' data also indicated that IMW might be more reflective of a horse's individual body shape than anything else.