Attentional bias is the tendency for a person's perception to be affected by selective factors in their attention. Attentional biases may explain an individual's failure to consider alternative possibilities when occupied with an existing train of thought. For example, cigarette smokers have been shown to possess an attentional bias for smoking-related cues around them, due to their brain's altered reward sensitivity. Attentional bias has also been associated with clinically relevant symptoms such as anxiety and depression.

A commonly studied experiment to test for attentional bias is one in which there are two variables, a factor (A) and a result (B). Both can be either present (P) or not present (N). This results in four possible combinations:

The four combinations can be shown in table form as follows:

A common question which follows the structure of the above experiment is: "Does God answer prayers?" Due to attentional bias, theists tend to say "yes". They focus on the present/present (A/B) cell, as their religious beliefs in a deity cause them to fixate on the occasions when they were given what they asked for, thus they use the justification: "Many times I've asked God for something, and he's given it to me." Similarly, due to attentional bias, atheists equally tunnel on data from the present/absent (A/B', A'/B) cells: "Has God ever given me something that I didn't ask for?" or "Have I asked God for something and didn't receive it?" This experiment too supports Smedslund's general conclusion that subjects tend to ignore part of the table depending on their specific attentional biases.

The scenarios can be illustrated below in a similar table to above:

When making decisions, attentional biases toward positive stimuli have been associated with numerous positive outcomes, such as increased social engagement, increased prosocial behaviour, decreased externalizing disorders, and decreased emotionally withdrawn behavior. In contrast, individuals with clinically relevant symptoms, such as anxiety disorder and chronic pain are shown to prioritize threat cues over reward cues. In one experiment, faces with varying valences were presented (neutral, threatening, and happy) with a forced-choice reaction time at two exposure durations, 500 and 1250msec. For individuals with high trait anxiety, there was strong evidence for an attentional bias favoring threatening facial expressions. Additionally, increased dysphoria correlated with the tendency to avoid happy faces. This tendency leads to a spiraling effect, as one will only see negative faces, which induces greater anxiety, which exacerbates the aforementioned tendency to avoid positive stimuli – a form of the vigilance-avoidance pattern.

Notably, there is also a difference in attention biases between anxious and depressed individuals. Word pairs were shown to the subjects, with a dot probe following a word of each pair (dot probe paradigm). One-half of the word pairs were presented on the subliminal level, and the other half were presented on the supraliminal level, and then the response time was measured. As expected, the anxious and depressed groups showed an attentional bias towards negative words compared to the normal control group. On a supraliminal level, the depressed group showed greater vigilance for threat stimuli than the anxious group. However, for subliminal threat stimuli, the anxious group showed a greater vigilance, which implies an anxiety-related bias on the subconscious level.

Research from the past two decades has established that addictive behaviour is strongly correlated to the attentional bias for substance-related cues, in how the latter characterizes the former. An example of this is smoking and smoking-related cues.