Since the earliest crash investigations, whiplash has been found to occur more often in women than men. This study addresses seat properties that may explain a reason for the higher rates in women, and changes in whiplash in general over the past two decades. Three exemplar seats were defined on the basis of seat stiffness (k) and frame rotation stiffness (j) for rearward occupant load. Stiff seats have k=40 kN/m and j=1.8 degrees /kN representing a foreign benchmark loaded by a male. One yielding seat had k=20 kN/m and j=1.4 degrees /kN simulating a high-retention seat (1997 Grand Prix) and another k=20 kN/m and j=3.4 degrees /kN simulating a 1980s to 1990s yielding seat (1990 Buick Park Avenue). Constant vehicle acceleration for 100 msec gave delta-V of 6, 10, 16, and 24 km/h. The one-dimensional model included a torso mass loading the seatback with flexible neck and head mass. Based on biomechanical data and scaling, neck stiffness was 5 kN/m and 3 kN/m for the male and female, respectively. Based on validation tests, seat stiffness was 25% less with the female. Occupant dynamics were simulated in a step-forward solution based on the differential displacement between the head, torso, and seat up to head restraint contact. Neck responses were 30% higher in the female than male through most of the rear impact and are proportional to (kF/mTF)/(kM/mTM), which is the ratio of seat stiffness divided by torso mass for the female and male. Neck displacements were higher with the stiff seat than the 1990 C car seat for both the female and male. They peaked at 10 km/h and dropped off for higher severity crashes due to the shorter time to head contact. Neck displacements were greater in the female than male for the lowest severity crashes with the stiff and 1990 C car seats, when displacement was scaled for equal tolerance. The female in 1997 W car seat had the lowest neck displacements. Stiff seats increased neck displacements over the yielding seats of the 1980s in rear crashes. The trend is similar in men and women, but early neck displacements are greater in women because of a higher ratio of seat stiffness to torso mass. This implies that seat stiffness is not sufficiently low in proportion to the female mass in comparison to males. The j and k seat properties influence neck biomechanics and occupant dynamics, but k is important in determining early response differences between males and females.