Hierarchies are one of the programmer's most durable tools. Much of the data inside programs, along with much of the code that manipulates it, is in hierarchical form. For this reason, many programmers present hierarchies (the implementation model) in user interfaces. Early menus, as we've seen, were hierarchical. But abstract hierarchies are very difficult for users to successfully navigate. This truth is often difficult for programmers to grasp because they themselves are so comfortable with hierarchies.
Most humans are familiar with hierarchies in their business and family relationships, but hierarchies are not natural concepts for most people when it comes to storing and retrieving arbitrary information. Most mechanical storage systems are simple, composed either of a single sequence of stored objects (like a bookshelf) or a series of sequences, one level deep (like a file cabinet). This method of organizing things into a single layer of groups is extremely common and can be found everywhere in your home and office. Because it never exceeds a single level of nesting, we call this storage paradigm monocline grouping.
Programmers are very comfortable with nested system where an instance of an object is stored in another instance of the same object. Most other humans have a very difficult time with this idea. In the mechanical world, complex storage systems, by necessity, use different mechanical form factors at each level: in a file cabinet, you never see folders inside folders or file drawers inside file drawers. Even the dissimilar nesting of folder-inside-drawer-inside-cabinet rarely exceeds two levels of nesting. In the current desktop metaphor used by most window systems, you can nest folder within folder ad infinitum. It's no wonder most computer neophytes get confused when confronted with this paradigm.
Most people store their papers (and other items) in a series of stacks or piles based on some common characteristic: The Acme papers go here; the Project M papers go there; personal stuff goes in the drawer. Donald Norman (1994) calls this a pile cabinet. Only inside computers do people put the Project M documents inside the Active Clients folder, which, in turn, is stored inside the Clients folder, stored inside the Business folder.
Computer science gives us hierarchical structures as tools to solve the very real problems of managing massive quantities of data. But when this implementation model is reflected in the manifest model presented to users, they get confused because it conflicts with their mental model of storage systems. Monocline grouping is so dominant outside the computer that interaction designers violate this model at their peril.
Monocline grouping is an inadeguate system for physically managing the large quantities of data we commonly find on computers, but that doesn't mean it isn't useful as a manifest model. The solution to this conundrum is to render the structure as the user imagines it - as monocline grouping - but to provide the search and access tools that only a deep hierarchical organization can offer. In other words, rather than forcing users to navigate deep, complex tree structures, give them tools to bring appropriate information to them.