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Platform designers create and implement incentive systems to encourage users to contribute content to online communities. This article examines the effect of a multidimensional incentive hierarchy in motivating users to engage in competitive and prosocial activities. Utilizing an external change observed in the data science community, Kaggle, and applying a quasi-experimental design, we compared users’ engagement levels before and after introducing a multidimensional incentive hierarchy. We found that implementing a multidimensional incentive system directed users from submitting answers to Kaggle competitions to participating in Kaggle’s online forum discussions. However, our additional analyses suggest that the most and the least motivated users may be less likely to be impacted by such incentives. 

Capturing material properties of real-world elastic solids is both challenging and highly relevant to many applications in computer graphics, robotics and related fields. We give a non-intrusive, in-situ and inexpensive approach to measure the nonlinear elastic energy density function of man-made materials and biological tissues. We poke the elastic object with 3d-printed rigid cylinders of known radii, and use a precision force meter to record the contact force as a function of the indentation depth, which we measure using a force meter stand, or a novel unconstrained laser setup. We model the 3D elastic solid using the Finite Element Method (FEM), and elastic energy using a compressible Valanis-Landel material that generalizes Neo-Hookean materials by permitting arbitrary tensile behavior under large deformations. We then use optimization to fit the nonlinear isotropic elastic energy so that the FEM contact forces and indentations match their measured real-world counterparts. Because we use carefully designed cubic splines, our materials are accurate in a large range of stretches and robust to inversions, and are therefore animation-ready for computer graphics applications. We demonstrate how to exploit radial symmetry to convert the 3D elastostatic contact problem to the mathematically equivalent 2D problem, which vastly accelerates optimization. We also greatly improve the theory and robustness of stretch-based elastic materials, by giving a simple and elegant formula to compute the tangent stiffness matrix, with rigorous proofs and singularity handling. We also contribute the observation that volume compressibility can be estimated by poking with rigid cylinders of different radii, which avoids optical cameras and greatly simplifies experiments. We validate our method by performing full 3D simulations using the optimized materials and confirming that they match real-world forces, indentations and real deformed 3D shapes. We also validate it using a Shore 00 durometer, a standard device for measuring material hardness. 

Kirchhoff-Love shells are commonly used in many branches of engineering, including in computer graphics, but have so far been simulated only under limited nonlinear material options. We derive the Kirchhoff-Love thin-shell mechanical energy for an arbitrary 3D volumetric hyperelastic material, including isotropic materials, anisotropic materials, and materials whereby the energy includes both even and odd powers of the principal stretches. We do this by starting with any 3D hyperelastic material, and then analytically computing the corresponding thin-shell energy limit. This explicitly identifies and separates in-plane stretching and bending terms, and avoids numerical quadrature. Thus, in-plane stretching and bending are shown to originate from one and the same process (volumetric elasticity of thin objects), as opposed to from two separate processes as done traditionally in cloth simulation. Because we can simulate materials that include both even and odd powers of stretches, we can accommodate standard mesh distortion energies previously employed for 3D solid simulations, such as Symmetric ARAP and Co-rotational materials. We relate the terms of our energy to those of prior work on Kirchhoff-Love thin-shells in computer graphics that assumed small in-plane stretches, and demonstrate the visual difference due to the presence of our exact stretching and bending terms. Furthermore, our formulation allows us to categorize all distinct hyperelastic Kirchhoff-Love thin-shell energies. Specifically, we prove that for Kirchhoff-Love thin-shells, the space of all hyperelastic materials collapses to two-dimensional hyperelastic materials. This observation enables us to create an interface for the design of thin-shell Kirchhoff-Love mechanical energies, which in turn enables us to create thin-shell materials that exhibit arbitrary stiffness profiles under large deformations. 

Scholars have long debated the relationship between morality and the market. Some argue that morality tempers market interests, while others argue that the market has its own moral order. Meanwhile, feminist scholars have argued that a false binary between altruism, family, and intimacy on the one hand, and the cold calculus of the market on the other, is based in gender ideologies. Norms around motherhood, in particular, emphasize self-sacrifice, love, and altruism in opposition to self-interested market logics. Commercial surrogacy blurs the line between family and commerce and is therefore an ideal setting for studying tensions between altruism and profit. Drawing on ethnographic research and interviews with 114 actors in the Mexican surrogacy industry, I demonstrate that treating altruism and commercialism as dichotomous can further market interests by preserving the moral palatability and profitability of the industry while perpetuating power asymmetries rooted in gender, race, class, and nationality between surrogate mothers and intended parents.