This argues against any kind of recent accretion or merger models. Bar formation makes the orbits in the bar more eccentric and aligns their principal axes without greatly affecting the motion perpendicular to the plane. The statistics for later types are not as conclusive, probably because of difficulties in classifying their generally smaller bulges. The mechanism discussed here also has its drawbacks. Associated with those questions is the issue of the stability of spiral disks to bar formation.
Most simulations start with an equilibrium disk unstable to bar formation.
But, that instability is so quick to act that it is uncertain whether such a disk would have formed in the first place. Recent simulations by Mihos et al. This scenario couples the accretion and bar buckling mechanisms. In addition, this scenario offers numerous opportunities for observers to test its validity.