A key assumption of Mental Model theory (Johnson-Laird & Byrne, 2002) is that additional models increase working memory load. Accordingly, reasoners should tend to reason with a minimal representation (i.e., Initial Model: IM) when overburdened. This has never been demonstrated. In three studies, we presented adult Ps with conditional inferences, and burdened processing by restricting the time allotted (5 s. or 7 s) and by varying the degree of arbitrariness of the conditionals. With familiar conditionals, a small but significant (6.25%, p<.02) number of participants showed patterns consistent with IM in the 5 s. condition. With arbitrary conditionals, this showed greater use of IM in both time conditions (20%, p<.001). Finally, arbitrary conditionals with explicit negations in both terms, showed a high number of IM patterns (26%, p<.001), which increased with time. These studies provide clear evidence that reasoners use IM reasoning, mostly as a function of arbitrariness of premises.