To be replicated, a meme must pass successfully through four subsequent stages: 1) assimilation by an individual, who thereby becomes a host of the meme; 2) retention in that individual's memory; 3) expression by the individual in language, behavior or another form that can be perceived by others; 4) transmission of the thus created message or meme vehicle to one or more other individuals. This last stage is followed again by stage 1, thus closing the replication loop. At each stage there is selection, meaning that some memes will be eliminated. Let us look in more detail at the mechanisms governing these four stages.
A successful meme must be able to "infect" a new host, that is, enter into its memory. Let us assume that a meme is presented to a potential new host. "Presented" means either that the individual encounters a meme vehicle, or that he or she independently discovers it, by observation of outside phenomena or by thought, i.e. recombination of existing cognitive elements. To be assimilated, the presented meme must be respectively noticed, understood and accepted by the host.
Noticing requires that the meme vehicle be sufficiently salient to attract the host's attention. Understanding means that the host recognizes the meme as something that can be represented in his or her cognitive system. The mind is not a blank slate on which any idea can be impressed. To be understood, a new idea or phenomenon must connect to cognitive structures that are already available to the individual. Finally, a host that has understood a new idea must also be willing to believe it or to take it serious. For example, although you are likely to understand the proposition that your car was built by little green men from Mars, you are unlikely to accept that proposition without very strong evidence. Therefore, you will in general not memorize it, and the meme will not manage to infect you.
The second stage of memetic replication is the retention of the meme in memory. By definition, memes must remain some time in memory, otherwise they cannot be called memes. The longer the meme stays, the more opportunities it will have to spread further by infecting other hosts. This is Dawkins's (1976) longevity characteristic for replicators.
Just like assimilation, retention is characterized by strong selection, which few memes will survive. Indeed, most of the things we hear, see or understand during the day are not stored in memory for longer than a few hours. Although you may have very clearly assimilated the news that the progressive liberal party won the Swaziland elections with 54% of the votes, you are unlikely to remember anything of this a week later--unless you live in Swaziland, perhaps. Retention will depend on how important the idea is to you, and how often it is repeated, either by recurrent perception or by internal rehearsal. All learning paradigms agree that experiences are encoded more strongly into memory by frequent reinforcement.
To be communicated to other individuals, a meme must emerge from its storage as memory pattern and enter into a physical shape that can be perceived by others. This process may be called "expression". The most obvious means of expression is speech. Other common means for meme expression are text, pictures, and behavior. Expression does not require the conscious decision of the host to communicate the meme. A meme can be expressed simply by the way somebody walks or manipulates an object, or by what he or she wears.
Some retained memes will never be expressed, for example because the host does not consider the meme interesting enough for others to know, uses it unconsciously without it showing up in his or her behavior, does not know how to express it, or wants to keep it secret. On the other hand, the host may be convinced that the meme is so important that it must be expressed again and again to everybody he or she meets.
To reach another individual, an expression needs a physical carrier or medium which is sufficiently stable to transmit the expression without too much loss or deformation. Speech, for example, uses sound to transmit an expression, while text will be transmitted through ink on paper or electrical impulses in a wire. The expression will take the form of a physical signal, modulating the carrier into a specific shape from which the original meme can be re-derived. This physical shape may be called the meme vehicle. For example, meme vehicles can be books, photographs, artefacts or CD-ROMs.
Selection at the transmission stage happens through either elimination of certain memes, when the vehicle is destroyed or gets corrupted before it is perceived by another individual, or through differential multiplication, when the vehicle is reproduced into many copies. For example, a manuscript may be put into the shredder or it may be turned into a book which is printed in thousands of copies. A radio communication may get lost because of noise, or it may be broadcasted to millions of listeners. Especially since the emergence of mass media, including the electronic network, the transmission stage is the one where the contrast between successful and unsuccessful memes is largest, and where selection may have the largest impact.
Heylighen F. (1998): "What makes a meme successful?", in: Proc. 16th Int. Congress on Cybernetics (Association Internat. de Cybernétique, Namur), p. 423-418.