Universal grammar is the theoretical framework that explains this innate capacity. It posits that all human languages share a fundamental underlying structure, a set of universal principles and parameters that govern their grammar. This underlying structure is what's believed to be innate – the "blueprint" for language that is hardwired into our brains.
Therefore, the connection is this:
* Innateness Hypothesis provides the *why*: It explains *why* children acquire language so rapidly and effortlessly, despite the complexity of language. It suggests the process isn't solely based on imitation and reinforcement (behaviorism), but on an inherent predisposition.
* Universal Grammar provides the *how*: It offers a concrete mechanism, a system of rules and principles (UG), that constitutes this innate capacity. It explains *how* this innate predisposition manifests in the actual acquisition of diverse languages. The idea is that children aren't learning language from scratch; they're essentially "setting" parameters within this pre-existing framework of UG to match the specific language they're exposed to.
In essence, the innateness hypothesis is the general claim that language acquisition is facilitated by innate factors, while universal grammar is a specific theory proposing the nature of those innate factors. They are two sides of the same coin, with UG providing a detailed explanation for the innateness hypothesis in the realm of language acquisition.