Hypodermic needle theory

The hypodermic needle theory was not based on research but rather on assumptions of the time about human nature.
People were assumed to be "uniformly controlled by their instincts and that they react more or less uniformly to whatever the media told to them" - Lowery & De Fleur, 1995

The "Hypodermic Needle Model" uses the same idea of the "shooting" paradigm. It suggests that the media injects its messages straight into the passive audience. This passive audience is immediately affected by these messages. The public essentially cannot escape from the media's influence, and is therefore considered a "sitting duck" - (Croteau, Hoynes 1997)

The hypodermic needle is meant to give a mental image of the direct infusion of a message into an individual. But it became more highly developed, and it became apparent that the media had selective influences on people. 

Example of the hypodermic needle theory:
The most famous incident often cited as an example for the hypodermic needle model was the 1938 broadcast of The War of the Worlds and the subsequent reaction of widespread panic among its American mass audience. However this incident actually sparked the research led by Paul Lazarsfeld and Herta Herzog, that would disprove the magic bullet or hypodermic needle theory. Hadley Cantril managed to show that reactions to the broadcast were in fact diverse and were largely determined by situational and attitudinal attributes of the listeners.

Lazarsfeld disproved the Magic Bullet theory and "Hypodermic Needle Model Theory" through elections studies. Lazarsfeld and colleagues executed the study by gathering research during the 1940 American election. Lazarsfeld discovered that the majority of the public remained unfazed by propaganda surrounding Roosevelt's campaign. Instead, interpersonal outlets proved more influential than the media. Lazarsfeld introduced the idea of the two step flow theory. 

This video explains the hypodermic needle theory well.