The electrothermal properties of the perovskite oxides barium titanate (BTO), lead titanate (PTO), and strontium titanate (STO) are computed near the temperatures of their ferroelectric and/or ferroelastic phase transitions. The computations are performed using a modified 2-4-6 Ginzburg-Landau-Devonshire polynomial as functions of applied electric field and temperature for mechanically free monodomain crystals and for epitaxial thin films subject to perfect lateral clamping. For BTO and PTO, which display weak first-order ferroelectric phase transitions at their Curie points, the application of a bias field exceeding the electrical critical point reduces the dependence of the electrocaloric (EC) response on temperature and automatically reduces its magnitude. Under conditions of perfect lateral clamping, the weak first-order phase change is transformed into second-order phase change. In this instance the electrical critical point is coincident with the Curie temperature and a lower bias field is required to produce a comparable reduction in temperature sensitivity. Comparison of the electrothermal behaviors of BTO and PTO with that computed for STO near the temperature of the second-order ferroelastic phase transition provides insight concerning the EC properties of ferroelectric solid solution systems wherein the Curie temperature and the first-order character of the paraelectric to ferroelectric transition both may change subject to a change in composition. The results illustrate how electrical and mechanical boundary conditions can be adjusted, in conjunction with composition, in altering the EC properties of ferroelectric materials selected for use in a particular temperature range.