Robert and Elizabeth Bjork's term for a study condition that feels slower and harder in the moment but IMPROVES long-term retention and transfer.

The Bjorks, memory researchers at UCLA, noticed a recurring paradox: the practice conditions that make you feel most fluent and confident often teach you the least, while conditions that introduce struggle produce the most durable learning. A difficulty is "desirable" when the extra effort it demands is precisely what strengthens the memory. The core desirable difficulties are spacing, interleaving, retrieval practice, generation, and varied practice — all of them share the same signature: they trade easy, satisfying performance now for better recall later.