According to WebMD, very few studies have been conducted to support the use of a firm mattress over a soft mattress. Trial and error should be used to determine which type of mattress is right for each person.
In a study conducted by Kim Bergholdt, DC, of Denmark's Funen Back Center, 160 patients with lower back problems were assigned three different types of beds. The beds used were a hard futon, a water bed and a body-conforming foam mattress. The patients that were able to complete the study reported that they preferred the water bed or the body-contour mattress over the hard futon. The study's results were hard to interpret because of the inability to classify patients based on the cause of their back pain and the number of participants that didn't complete the study.