An invention of the Information Age, prefilters were originally intended to make sure that people only heard music they liked. A prefilter monitored personal reactions to certain types of music, then used evolutionary algorithms to determine what songs the individual liked most. This was relatively harmless, but later prefilteres monitored reactions to news. Soon people were only hearing news that they wanted to hear, and never had to have the unpleasantness of encountering an opinion that they didn't already agree with. This has some unfortunate side effects on social cohesion and consensus-building, though of course news of this was slow to spread to those who found it upsetting. Most societies and polities today place restrictions on the use of prefilters, or prohibit them entirely.