Do Social Media Shrink Our Brain?

Start considering that (A) Tamir and Mitchell 2012 show sound evidence that when we talk about our own personal experiences we activate relevant reward processes at the neural and cognitive level, and this is crucial in explaining why we talk so much about ourselves (30-40% on average, and ~80% on social media as Twitter).
Then consider that (B) Kanai et al. 2011 found significant positive correlations between the size of grey matter and the number of "friends" in social media (Facebook) networks.
Finally recall that (C) Fowler et al. 2009 discovered that genetic factors account for a relevant part of the number of social ties (46% of the variation of in-degree in population). That is to say that the size of our immediately accessible social network is largely a heritable trait.

Now, if we sum to A, B and C the "plausible" assumptions that being too much self-referential has a detrimental effect on the number of our social relationships and that online social media allows to increase the number of active social relationships we can maintain, should we conclude that humanity is condemned to devolve its brain size due to an intense usage of online social media?


Find your answer here, with a simple agent-based model:

The model has been developed in NetLogo 5.0.1 and it is downloadable here.
How the model works:
  • there are 100 agents, randomly located in a finite space;
  • agents have a finite life (randomly drawn at their birth between 20 and 100 time steps) and, when they die, they give birth to a new agent in the same location and with a new random color;
  • children, when entering the simulation (i.e., at the first time step of their life), inherit a part (46%) of their parental social network, and establish new social relationships according to their spatial location: the number of relationships they have at the end of their first year of life is strictly dependent on the percentage of communication they do on online social media (if they use only online social media for their social relationships, they can afford the double of friends);
  • each time step a link represents a communication from an agent to another, and given the probability of interacting on online social media and the probabilities of talking about personal experiences in communications (either offline or online), the communication can be about self experiences or other stuff;
  • the agent receiving a self-centered communication probabilistically decides to terminate the social relationship with the friend according to the tolerance parameter;
Why the model must be taken with care:
  • the resulting world is too much simplistic and with a social structure almost completely static: agents are condemned to establish social relationships only at the beginning of their life and with their spatially closest peers; 
  • the modeling of the data about heritability is made as if the parameter of 46% refers to inheriting social connections and not to how much genetics explains the variability of the values of degree in the population;
  • besides the fact that the brain is not only made by grey matter, evidence suggests that human brain size has been shrinking for the last 10,000 years (Hawks 2011).
What the model says:
  • higher tolerance levels increase social relationships and thus grey matter size;
  • an increase in the usage of online social media increases personal satisfaction only if we are highly tolerant in listening to others talking about their own personal experiences, and it does not affect the size of grey matter in our brains;
  • with low levels of tolerance about others' focus-on-self, an increase in the usage of online social media has detrimental effects both on grey matter size and personal satisfaction.

PS: this post is a provocative joke, but the referred science (reported below) is very serious.

D. I. Tamir and J. P. Mitchell, Disclosing information about the self is intrinsically rewarding, PNAS 2012
Abstract
Humans devote 30–40% of speech output solely to informing others of their own subjective experiences. What drives this propensity for disclosure? Here, we test recent theories that individuals place high subjective value on opportunities to communicate their thoughts and feelings to others and that doing so engages neural and cognitive mechanisms associated with reward. Five studies provided support for this hypothesis. Self-disclosure was strongly associated with increased activation in brain regions that form the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. Moreover, individuals were willing to forgo money to disclose about the self. Two additional studies demonstrated that these effects stemmed from the independent value that individuals placed on self-referential thought and on simply sharing information with others. Together, these findings suggest that the human tendency to convey information about personal experience may arise from the intrinsic value associated with self-disclosure.

R. Kanai, B. Bahrami, R. Roylance, and G. Rees, Online social network size is reflected in human brain structure, Proc R Soc B 2011
Abstract
The increasing ubiquity of web-based social networking services is a striking feature of modern human society. The degree to which individuals participate in these networks varies substantially for reasons that are unclear. Here, we show a biological basis for such variability by demonstrating that quantitative variation in the number of friends an individual declares on a web-based social networking service reliably predicted grey matter density in the right superior temporal sulcus, left middle temporal gyrus and entorhinal cortex. Such regions have been previously implicated in social perception and associative memory, respectively. We further show that variability in the size of such online friendship networks was significantly correlated with the size of more intimate real-world social groups. However, the brain regions we identified were specifically associated with online social network size, whereas the grey matter density of the amygdala was correlated both with online and real-world social network sizes. Taken together, our findings demonstrate that the size of an individual's online social network is closely linked to focal brain structure implicated in social cognition.

J. H. Fowler, C. T. Dawes, and N. A. Christakis, Model of genetic variation in human social networks, PNAS 2009
Abstract
Social networks exhibit strikingly systematic patterns across a wide range of human contexts. Although genetic variation accounts for a significant portion of the variation in many complex social behaviors, the heritability of egocentric social network attributes is unknown. Here, we show that 3 of these attributes (in-degree, transitivity, and centrality) are heritable. We then develop a “mirror network” method to test extant network models and show that none account for observed genetic variation in human social networks. We propose an alternative “Attract and Introduce” model with two simple forms of heterogeneity that generates significant heritability and other important network features. We show that the model is well suited to real social networks in humans. These results suggest that natural selection may have played a role in the evolution of social networks. They also suggest that modeling intrinsic variation in network attributes may be important for understanding the way genes affect human behaviors and the way these behaviors spread from person to person.

J. Hawks, Selection for smaller brains in Holocene human evolutionarXiv 2011
Abstract
Background: Human populations during the last 10,000 years have undergone rapid decreases in average brain size as measured by endocranial volume or as estimated from linear measurements of the cranium. A null hypothesis to explain the evolution of brain size is that reductions result from genetic correlation of brain size with body mass or stature. 
Results: The absolute change of endocranial volume in the study samples was significantly greater than would be predicted from observed changes in body mass or stature. 
Conclusions: The evolution of smaller brains in many recent human populations must have resulted from selection upon brain size itself or on other features more highly correlated with brain size than are gross body dimensions. This selection may have resulted from energetic or nutritional demands in Holocene populations, or to life history constraints on brain development.

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