Short answers, pulled from the story.
Heterogeneous agents refers to a macroeconomic model where consumers are assumed to differ from one another rather than acting identically. This concept acknowledges that individuals have unique spending habits and distinct behaviors instead of following standard assumptions that all people act the same way.
M. Arellano published this finding in 2003 within Chapter 2 of Panel Data Econometrics. The text states that hidden factors can distort conclusions drawn by economists analyzing dependent and independent variables if they remain unobserved yet correlate with observed data.
Individual demand aggregates to market demand only if preferences follow the Gorman polar form. This condition requires linear and parallel Engel curves across every consumer, meaning summing individual choices fails to represent the whole accurately when preferences deviate from this shape.
Models with adaptive expectations fall into the category of agent-based computational economics or ACE where participants learn through trial and error. Conversely dynamic stochastic general equilibrium or DSGE applies when agents hold rational expectations based on perfect foresight of available information.
Heathcote Storesletten and Violante published convenient functional form assumptions in 2009 within AEJ Macro. Their work allows for some dimensions of heterogeneity while maintaining an analytical solution for general equilibrium.