Evolutionary psychologist Claudio Tennie, Tomasello, and Josep Call call this the "cultural ratchet" and they describe it as being unique to human culture. Receptors which initiate cell fate transduction cascades, in early embryo development, exhibit a ratchet effect in response to morphogen concentrations. The low receptor occupancy permits increases in receptor occupancy which alter the cell fate, but the high receptor affinity does not allow ligand dissociation leading to a cell fate of a lower concentration. The ratchet effect is reflected in the Collingridge dilemma. The ratchet effect can be seen in long-term trends in the production of many consumer goods. Year by year, automobiles gradually acquire more features. Competitive pressures make it hard for manufacturers to cut back on the features unless forced by a true scarcity of raw materials (e.g., an oil shortage that drives costs up radically). University textbook publishers gradually get "stuck" in producing books that have excess content and features. In software development, products which compete often will use specification lists of competitive products to add features, presuming that they must provide all of the features of the competitive product, plus add additional functionality. This can lead to "feature creep" in which it is considered necessary to add all of a competitor's features whether or not customers will use them. Airlines initiate frequent-flyer programs that become ever harder to terminate. Successive generations of home appliances gradually acquire more features; new editions of software acquire more features; and so on. With all of these goods, there is ongoing debate as to whether the added features truly improve usability, or simply increase the tendency for people to buy the goods. The term was included by the MAI Negotiating Group in the 1990s as the essence of a device to enforce legislative progress toward "free trade" by preventing legislative rollback with the compulsory assent of governments as a condition of participation. Rollback is the liberalisation process by which the reduction and eventual elimination of nonconforming measures to the MAI would take place. It is a dynamic element linked with standstill, which provides its starting point. Combined with standstill, it would produce a "ratchet effect", where any new liberalisation measures would be "locked in" so they could not be rescinded or nullified over time.