As an issue in motor development, the nature vs. nurture debate won’t seem to die, despite Newell’s (1986) insight that it is a “false dichotomy,” since both hereditary and environmental “causes” are deterministic, not probabilistic, explanations (Haywood & Getchell, 2001; Gabbard, 2004). Nowhere has the nature-nurture controversy been stronger than in the study of the development of overarm throwing. Incontrovertible evidence exists for strong gender differences in throwing for quantitative and qualitative measures (Halverson, Roberton, & Langendorfer, 1982; Thomas & French, 1985) leading investigators to conclude that either biology or experience produce these differences. The current study proposed investigating the overarm throw for force using Platt’s (1964) strong inference technique to disprove one or more alternative hypotheses about sources of throwing differences. Twenty college-aged students (n=12 males; n=8 females) gave informed consent to throw a total of 40 trials in four randomized blocks of ten trials. Conditions included throwing with the preferred hand and non-preferred hand 10 times each followed by 10 more throws with each hand after receiving simple instructions about forearm pronation (for non-preferred hand throws) and forearm supination (for preferred hand throws). The authors digitally videotaped all throws from both side and rear views at a camera rate of 120Hz. We also recorded resultant ball velocities with a rear positioned radar gun (JUGS). Each throw was categorized by one or more of the investigators using Roberton’s (1977, 1984) component developmental sequences for trunk, humerus, and forearm actions after having established inter- and intra-rater objectivity of P³.90 exact agreement and k³.75. We used a 2x2x2 factorial ANOVA (gender x hand x treatment) with repeated measures on the last two factors to test for ball velocity differences. We tested the ordinal developmental sequence data using Chi-square analyses. As expected, significant main effects for gender and hand existed for both ball velocity and developmental sequence data. Significant interactions for gender x hand and hand x treatment occurred for the velocity data. These results supported a probabilistic rather than a deterministic interpretation of gender differences and developmental change in throwing.