The first mouse lived in a house that contained, along with furniture and other household goods and possessions, a lever and a hole in the wall from which food was delivered. Each time the mouse pressed the lever he would receive a tasty morsel of his favourite food. The mouse understood that, when he was hungry, all he had to do was press the lever and food would arrive via the hole. The mouse took great comfort in the predictability of his access to food and only pressed the lever when he was hungry.
The second mouse lived in a similar house, also containing a lever and a hole in the wall from which food was delivered. Unfortunately, the lever in his house was faulty and delivered food on an inconsistent basis when he pressed it, such that he might only receive food via the hole on the first, fifth, seventh, or even the eleventh time he pressed the lever. This mouse learnt that he could not always rely on the lever and that he had to press the lever many times, and even when he was not actually hungry, in order to ensure that he would have food. Even after his lever was fixed he found it difficult to stop pressing it frequently and displayed a habit of storing up food.
The third mouse also lived in a similar house, containing a lever and a hole in the wall from which food was to be delivered. However, the lever in his house did not work at all. He soon learnt that he could not rely on the lever and would have to develop other ways of gaining access to food. This belief persisted, even when he moved to a new home with a fully-functioning lever.
Source: Pearce, C. A Short Introduction to Attachment and Attachment Disorder. London: Jessica Kingsley, 2009