Adult ignorance is usually highly motivated. The desire to remain ignorant is far stronger than the desire to become knowledgeable, skillful, informed or aware. The desires differ and the motivations to remain ignorant can be categorized. When I've taught instructional design, I've used this schema to explain why the need for "training" is so often misdiagnosed and the outcomes amount to "snake oil cures" (90% zero or negative skill transfer rate). This model is also useful to see beyond the evidence of a "barrier to new concepts" and to discern the underlying motivations. With all this, it becomes possible to challenge the design premise that "delivering content will seem useful and valuable to the adult learners".
Retaliatory ignorance: Remaining ignorant is a "great way" to get even with adversaries. Not knowing something can "get their goat" or "pull their chain". Ignorance can function as payback for putdowns, stereotyping or cheap shots. Remaining in the dark can "trip up a bully and pull the rug out from a power tripper". How we've been mismanaged creates a barrier to new concepts.Structural ignorance: Comfort zones and success routines depend on the denial of information that creates "cognitive dissonance". Any established paradigm dismisses contradictory evidence as a threat to it's consistency and validity. Pride in one's own superiority invalidates the viewpoints which see underlying insecurity and inferiority in the self righteous claims. Advisory panels succumb to groupthink to defend themselves against disruptive information or internal conflicts. These minds are closed to protect their false sense of security from "sticking to their own kind". What we already know creates a barrier to new concepts.
Ostentatious ignorance: Peer pressure often aligns with remaining ignorant. Becoming knowledgeable or competent gets framed as "selling out or kissing up". Then it pays to act clueless both for show and to avoid ostracization. Appearing stupid gains validation from the marginalized subculture of disenfranchised citizens. The incentives in the social system favor the peer validation because there are next-to-none coming from the educational delivery system. Who gives us a sense of membership creates a barrier to new concepts.Hypnotic ignorance: Schooling, grading, parenting and classmates can give us the impression we cannot learn. We discover we are damaged. Our memory, concentration or ability to comprehend new information does not work like it should. We don't digest, internalize or assimilate what we try to learn. We have fallen under an evil spell or become captivated by an oppressive narrative. We believe what we have been told about being defective, deviant or deficient. We have internalized abuse in a way that disables our natural curiosity, creativity and connecting proclivities. How we've been framed creates a barrier to new concepts.
Honest ignorance: Sometimes we really don't know something and what to clear that up right away. We have the question and were wondering about that just now. If the information was previously available, we didn't notice it because we were not ready for it or did not need it yet. But now is the time. Our minds are like parachutes - working now that they are open. How we've been respected removes the barrier to new concepts.