Posted Yesterday, 15:04
If you have 5-5 in the minors, unless your hand is very strong, you will often want to keep 3nt in the picture as a possible contract. This is a problem with bidding 3♦ followed by 4♣, which should be reserved for really strong hands (such that slam in a minor is likely and bypassing 3nt is not such a big cost). With a relatively minimum game force, it's often better to double, giving up on 5m/6m on an eight-only-card fit if partner has the required major controls for 3nt.
With 5-5 (or 6-6!) in the majors, it's different. You are generally willing to bypass 3nt, since 4M is likely to be a fine contract the vast majority of the time. Doubling will only help you if partner has FOUR card support for one of the majors, whereas bidding 3♠ followed by an appropriate number of hearts has a strong chance to find a fit when partner has only three-support.
Another thought is that partner occasionally leaves a double in when it looks like a misfit, and this is likely to be pretty poor when you have 6-6 majors (partner could easily have 3-3 and think there's no fit, since your double will frequently have 4-4 majors only, or possibly even 4-3 or the like since it's an awkward sequence).
Adam W. Meyerson
a.k.a. Appeal Without Merit