Evaluative conditioning (also referred to as evaluative learning) concerns how we can come to like or dislike something through an association (association of ideas).
If something that we have no strong feelings towards(such as an unfamiliar person, object, or picture, and so on) appears to us with something that we strongly dislike (such as a very unpleasant odour) then our feelings to that once innocuous item can change—we can come to dislike that thing too. The same can happen when something is paired with something else that we strongly like—we can come to like that item more.
An example that most people are likely to have experienced is a person's name. Most people probably have names that they strongly dislike. Some of these may be the names of people from their past who they disliked a lot. It is quite remarkable how strongly we can feel towards certain names, as anyone who has ever had to name a child will tell you. The strong feelings towards a person's name is acquired by the associatin of the name with feelings towards the person with that name.
An interesting question for research is the process by which feelings towards the previously neutral item have changed. Two possibilities are that:
(1) If a neutral item (N) is paired with a disliked item (D), then feelings towards N might change because when you think of or see N you think of D. Hence, you don't like N because it reminds you of D.
(2) If item N is paired with disliked item D, then the feelings towards N actually change regardless of whether it reminds you of D.
The first possibility is the signal learning hypothesis. N is merely a signal for D (in the sense that if you see N then it reminds you of D). In this case, you dislike N because some internal representation of D becomes activated in your mind (either consciously or unconsciously).
The second possibility implies an intrinsic learning of affect for N. For example, people can like particular shoes, chairs, kitchen units, paintings, flowers, sounds, ties, handbags, cars, colour schemes, not because they are necessarily reminded of something else when looking at or handling the item, but rather because the item initiates its own feelings in you.
The real possibility is that people can acquire a liking or a disliking of something through evaluative learning (ie, through an association) but then go on to have no recollection of how they acquired such a feeling (that is, they are unable to recall the nature of the original association). There is a lot of experimental evidence that suggests this to be the case.