Hypodermic Needle Theory -
Dating from the 1920s, this theory was the first attempt to explain how mass audiences might react to mass media. It is a crude model (see picture!) and suggests that audiences passively receive the information transmitted via a media text, without any attempt on their part to process or challenge the data. Don't forget that this theory was developed in an age when the mass media were still fairly new - radio and cinema were less than two decades old. Governments had just discovered the power of advertising to communicate a message, and produced propaganda to try and sway populaces to their way of thinking. This was particularly rampant in Europe during the First World War (look at some posters here) and its aftermath.
Basically, the Hypodermic Needle Model suggests that the information from a text passes into the mass consciouness of the audience unmediated, ie the experience, intelligence and opinion of an individual are not relevant to the reception of the text. This theory suggests that, as an audience, we are manipulated by the creators of media texts, and that our behaviour and thinking might be easily changed by media-makers. It assumes that the audience are passive and heterogenous. This theory is still quoted during moral panics by parents, politicians and pressure groups, and is used to explain why certain groups in society should not be exposed to certain media texts (comics in the 1950s, rap music in the 2000s), for fear that they will watch or read sexual or violent behaviour and will then act them out themselves.
Here is a Youtube video that will help explain the theory better:
