Extrusion is one of the main processes in the thermoplastic and rubber industry. However, various defects and flow instabilities occur not only to limit the production rates but also to influence the appearance and the quality of polymer extrudate products. A melt instability called gross melt fracture is often observed when the extrusion rate is increased beyond a critical value. This instability is characterized by a helical or periodic distortion evolving toward chaotic distortion. Because gross melt fracture limits the production rate, much attention has been devoted to predicting when it will occur. However, there is still no general understanding of the origin of this phenomenon. In this work, we focused on two series of polystyrene (monodisperse IP<1.5 and polydisperse IP > 2) to understand the physical mechanisms at the origin of this phenomenon. The studies of the critical shear rate for the onset of gross melt fracture show that this defect appears when the residence time of the melt in the die is lower than its reptation time for the monodisperse polymers or the relaxation time of the tube caused by constraint release for the polydisperse ones. This result shows the importance of the shape of the molecular weight distribution and more precisely its low molecular weight tail on the onset of the gross melt fracture defect. That gives a tool to postpone or to eliminate this defect.