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Adults can calculate probabilities by running simulations and calculating proportions of each outcome. How does this ability develop? We developed a method that lets us bring computational modeling to bear on this question. A study of 40 adults and 31 4-year-olds indicates that unlike adults, many 4-year-olds use a single simulation to estimate probability distributions over simulated possibilities. We also implemented the 3-cups task, an established test of children’s sensitivity to possibilities, in a novel format. We replicate existing 3-cups results. Moreover, children who our model categorized as running a single simulation on our novel task show a signature of running a single simulation in the 3-cups task. This signature is not observed in children who were categorized as running multiple simulations. This validates our model and adds to the evidence that about half of 4-year-olds don’t evaluate multiple candidates for reality in parallel. 

Understanding the orientation dynamics of anisotropic colloidal particles is important for suspension rheology and particle self-assembly. However, even for the simplest case of dilute suspensions in shear flow, the orientation dynamics of non-spherical Brownian particles are poorly understood. Here we analytically calculate the time-dependent orientation distributions for non-spherical axisymmetric particles confined to rotate in the flow–gradient plane, in the limit of small but non-zero Brownian diffusivity. For continuous shear, despite the complicated dynamics arising from the particle rotations, we find a coordinate change that maps the orientation dynamics to a diffusion equation with a remarkably simple ratio of the enhanced rotary diffusivity to the zero shear diffusion: $$D_{eff}^{r}/D_{0}^{r}=(3/8)(p-1/p)^{2}+1$$ , where $$p$$ is the particle aspect ratio. For oscillatory shear, the enhanced diffusion becomes orientation dependent and drastically alters the long-time orientation distributions. We describe a general method for solving the time-dependent oscillatory shear distributions and finding the effective diffusion constant. As an illustration, we use this method to solve for the diffusion and distributions in the case of triangle-wave oscillatory shear and find that they depend strongly on the strain amplitude and particle aspect ratio. These results provide new insight into the time-dependent rheology of suspensions of anisotropic particles. For continuous shear, we find two distinct diffusive time scales in the rheology that scale separately with aspect ratio $$p$$ , as $$1/D_{0}^{r}p^{4}$$ and as $$1/D_{0}^{r}p^{2}$$ for $$p\gg 1$$ . For oscillatory shear flows, the intrinsic viscosity oscillates with the strain amplitude. Finally, we show the relevance of our results to real suspensions in which particles can rotate freely. Collectively, the interplay between shear-induced rotations and diffusion has rich structure and strong effects: for a particle with aspect ratio 10, the oscillatory shear intrinsic viscosity varies by a factor of $${\approx}2$$ and the rotational diffusion by a factor of $${\approx}40$$ .