The primary purpose of this study was to examine the within-group variability that exists among low-income fathers by exploring their relationship with their child's mother. The within-group variability was assessed using item response theory (IRT) methodology. The current study used baseline data from the Fragile Families and Child Well-Being Study. A total of 1,025 fathers completed 10 items that addressed common relationship issues. The study found 3 classes of fathers, labeled the ordinary father, the overwhelmed father, and the aggressive father. The study concluded that within-group diversity does exist among young, low-income fathers, and the study provided important insights about the quality of the father-mother relationship. The study also found evidence supporting the use of IRT methodology when examining within-group differences. Limitations and suggestions for future research are also discussed.