Life-forms can transmit information both vertically (from parent to child, via replication of genes) and horizontally (through viruses and other means). Malcolm Gladwell wrote, “A meme is an idea that behaves like a virus–that moves through a population, taking hold in each person it infects.” Memes can replicate vertically or horizontally within a single biological generation. They may also lie dormant for long periods of time. Memes spread by the behaviors that they generate in their hosts. Imitation counts as an important characteristic in the propagation of memes. Imitation often involves the copying of an observed behaviour of another individual, but memes may transmit from one individual to another through a copy recorded in an inanimate source, such as a book or a musical score. McNamara has suggested that memes can be thereby classified as either internal or external memes, (i-memes or e-memes). Researchers have observed memetic copying in just a few species on Earth, including hominids, dolphins and birds (that learn how to sing by imitating their parents or neighbors).
Some commentators have likened the transmission of memes to the spread of contagions. Social contagions such as fads, hysteria, copycat crime, and copycat suicide exemplify memes seen as the contagious imitation of ideas. Observers distinguish the contagious imitation of memes from instinctively contagious phenomena such as yawning and laughing, which they consider innate (rather than socially learned) behaviors.
Aaron Lynch described seven general patterns of meme transmission, or “thought contagion”:
Quantity of parenthood: an idea that influences the number of children one has. Children respond particularly receptively to the ideas of their parents, and thus ideas that directly or indirectly encourage a higher birthrate will replicate themselves at a higher rate than those that discourage higher birthrates.
Efficiency of parenthood: an idea that increases the proportion of children who will adopt ideas of their parents. Cultural separatism exemplifies one practice in which one can expect a higher rate of meme-replication—because the meme for separation creates a barrier from exposure to competing ideas.
Proselytic: ideas generally passed to others beyond one’s own children. Ideas that encourage the proselytism of a meme, as seen in many religious or political movements, can replicate memes horizontally through a given generation, spreading more rapidly than parent-to-child meme-transmissions do.
Preservational: ideas that influence those that hold them to continue to hold them for a long time. Ideas that encourage longevity in their hosts, or leave their hosts particularly resistant to abandoning or replacing these ideas, enhance the preservability of memes and afford protection from the competition or proselytism of other memes.
Adversative: ideas that influence those that hold them to attack or sabotage competing ideas and/or those that hold them. Adversative replication can give an advantage in meme transmission when the meme itself encourages aggression against other memes.
Cognitive: ideas perceived as cogent by most in the population who encounter them. Cognitively transmitted memes depend heavily on a cluster of other ideas and cognitive traits already widely held in the population, and thus usually spread more passively than other forms of meme transmission. Memes spread in cognitive transmission do not count as self-replicating.
Motivational: ideas that people adopt because they perceive some self-interest in adopting them. Strictly speaking, motivationally transmitted memes do not self-propagate, but this mode of transmission often occurs in association with memes self-replicated in the efficiency parental, proselytic and preservational modes.