title: Longitudinal analysis of large social networks: Estimating the effect of health traits on changes in friendship ties Dynamic social networks promote cooperation in experiments with humans Cooperative behavior cascades in human social networks Food-Sharing Networks in Lamalera, Indonesia Emergence and Robustness of a Community Discussion Network on Mercury Contamination and Health in the Brazilian Amazon Social networks and infant feeding in Oaxaca, Mexico. Food-Sharing Networks in Lamalera, Indonesia: Status, Sharing, and Signaling. Raising the Level of Analysis of Food-Borne Outbreaks: Food-Sharing Networks in Rural Coastal Ecuador A 61-million-person experiment in social influence and political mobilization
abstract: We develop novel mixed effects models to examine the role of health traits on the status of peoples ' close friendship nominations in the Framingham Heart Study . The health traits considered are both mutable ( body mass index ( BMI ) , smoking , blood pressure , body proportion , muscularity , and depression ) and , for comparison , basically immutable ( height , birth order , personality type , only child , and h and edness ) ; and the traits have varying degrees of observability . We test the hypotheses that existing ties ( i.e. close friendship nominations ) are more likely to dissolve between people with dissimilar ( mutable and observable ) health traits whereas new ties are more likely to form between those with similar ( mutable and observable ) traits while controlling for persons ' age , gender , geographic separation , and education . The mixed effects models contain r and om effects for both the nominator ( ego ) and nominated ( alter ) persons in a tie to account for the fact that people were involved in multiple relationships and contributed observations at multiple exams . Results for BMI support the hypotheses that people of similar BMI are less likely to dissolve existing ties and more likely to form ties , while smoker to non-smoker ties were the least likely to dissolve and smoker to smoker ties were the most likely to form . We also vali date d previously known findings regarding homophily on age and gender , and found evidence that homophily also depends upon geographic separation . Copyright © 2011 John Wiley & Sons , Human population s are both highly cooperative and highly organized . Human interactions are not r and om but rather are structured in social networks . Importantly , ties in these networks often are dynamic , changing in response to the behavior of one 's social partners . This dynamic structure permits an important form of conditional action that has been explored theoretically but has received little empirical attention : People can respond to the cooperation and defection of those around them by making or breaking network links . Here , we present experimental evidence of the power of using strategic link formation and dissolution , and the network modification it entails , to stabilize cooperation in sizable groups . Our experiments explore large-scale cooperation , where subjects ’ cooperative actions are equally beneficial to all those with whom they interact . Consistent with previous research , we find that cooperation decays over time when social networks are shuffled r and omly every round or are fixed across all rounds . We also find that , when networks are dynamic but are up date d only infrequently , cooperation again fails . However , when subjects can up date their network connections frequently , we see a qualitatively different outcome : Cooperation is maintained at a high level through network rewiring . Subjects preferentially break links with defectors and form new links with cooperators , creating an incentive to cooperate and leading to substantial changes in network structure . Our experiments confirm the predictions of a set of evolutionary game theoretic models and demonstrate the important role that dynamic social networks can play in supporting large-scale human cooperation Theoretical models suggest that social networks influence the evolution of cooperation , but to date there have been few experimental studies . Observational data suggest that a wide variety of behaviors may spread in human social networks , but subjects in such studies can choose to befriend people with similar behaviors , posing difficulty for causal inference . Here , we exploit a seminal set of laboratory experiments that originally showed that voluntary costly punishment can help sustain cooperation . In these experiments , subjects were r and omly assigned to a sequence of different groups to play a series of single-shot public goods games with strangers ; this feature allowed us to draw networks of interactions to explore how cooperative and uncooperative behaviors spread from person to person to person . We show that , in both an ordinary public goods game and in a public goods game with punishment , focal individuals are influenced by fellow group members ’ contribution behavior in future interactions with other individuals who were not a party to the initial interaction . Furthermore , this influence persists for multiple periods and spreads up to three degrees of separation ( from person to person to person to person ) . The results suggest that each additional contribution a subject makes to the public good in the first period is tripled over the course of the experiment by other subjects who are directly or indirectly influenced to contribute more as a consequence . These results show experimentally that cooperative behavior cascades in human social networks Exponential r and om graph modeling ( ERGM ) is used here to test hypotheses derived from human behavioral ecology about the adaptive nature of human food sharing . Respondents in all ( n = 317 ) households in the fishing and sea-hunting village of Lamalera , Indonesia , were asked to name those households to whom they had more frequently given ( and from whom they had more frequently received ) food during the preceding sea-hunting season . The responses were used to construct a social network of between-household food-sharing relationships in the village . The results show that kinship , proximity , and reciprocal sharing all strongly increase the probability of giving food to a household . The effects of kinship and distance are relatively independent of each other , although reciprocity is more common among residentially and genealogically close households . The results show support for reciprocal altruism as a motivation for food sharing , while kinship and distance appear to be important partner-choice criteria Information exchanges , debates , and negotiations through community social networks are essential to ensure the sustainability of the development process initiated in participatory research . The authors analyze the structural properties and robustness of a discussion network about mercury issues in a community in the Brazilian Amazon involved in a participatory research aim ed at reducing exposure to the pollutant . Most of the villagers are connected in a large network and are separated from other individuals by few intermediaries . The structure of the discussion network displays resilience to the r and om elimination of villagers but shows vulnerability to the removal of one villager who has been a long-term collaborator of the project . Although the network exhibits a structure likely to favor an efficient flow of information , results show that specific actions should be taken to stimulate the emergence of a pool of opinion leaders and increase the redundancy of discussion channels The health benefits of delaying the introduction of complementary foods to infants ' diets are widely known . Many studies have shown that mothers with the support of close social network members are more compliant with medical recommendations for infant feeding . In our study , we examine the effects of a broader spectrum of network members ( 40 people ) on mothers ' infant feeding decisions . The survey was conducted in Oaxaca , Mexico as part of a follow-up to a nationwide Mexican Social Security Institute survey of infant health . Sixty mothers were interviewed from a stratified r and om sample of the original respondents .  Multivariate tests were used to compare the efficacy of network-level variables for predicting the introduction of 36 foods into infants ' diets , when compared with respondent-level variables . The study yields four findings . First , network-level variables were better predictors of the timing of food introduction than socio-demographic variables . Second , mothers with more indigenous networks delayed the introduction of some grains ( oatmeal , cereal , noodle soup , rice ) and processed pork products ( sausage and ham ) to the infant 's diet longer than mothers with less indigenous networks . Third , mothers who had stronger ties to their networks delayed the introduction of rice and processed pork products ( sausage and ham ) to the infant 's diet longer than mothers who had weaker ties to their networks . Fourth , mothers who heeded the advice of distant network members introduced some grains ( rice and cereal )  earlier than mothers who did not heed the advice of distant network members Costly signaling has been proposed as a possible mechanism to explain food sharing in foraging population s. This sharing-as-signaling hypothesis predicts an association between sharing and status . Using exponential r and om graph modeling ( ERGM ) , this prediction is tested on a social network of between-household food-sharing relationships in the fishing and sea-hunting village of Lamalera , Indonesia . Previous analyses ( Nolin 2010 ) have shown that most sharing in Lamalera is consistent with reciprocal altruism . The question addressed here is whether any additional variation may be explained as sharing-as-signaling by high-status households . The results show that high-status households both give and receive more than other households , a pattern more consistent with reciprocal altruism than costly signaling . However , once the propensity to reciprocate and household productivity are controlled , households of men holding leadership positions show greater odds of unreciprocated giving when compared to households of non-leaders . This pattern of excessive giving by leaders is consistent with the sharing-as-signaling hypothesis . Wealthy households show the opposite pattern , giving less and receiving more than other households . These households may reciprocate in a currency other than food or their wealth may attract favor-seeking behavior from others . Overall , status covariates explain little variation in the sharing network as a whole , and much of the sharing observed by high-status households is best explained by the same factors that explain sharing by other households . This pattern suggests that multiple mechanisms may operate simultaneously to promote sharing in Lamalera and that signaling may motivate some sharing by some individuals even within sharing regimes primarily maintained by other mechanisms Objectives : Consuming contaminated food is a well-documented individual-level risk factor for diarrheal disease . The sharing of food also influences the distribution of diarrheal disease risk through a community and region . Underst and ing this social process at a population level is therefore an important dimension of risk not captured by st and ard individual-level analyses . We examined social networks related to food-sharing in rural villages at 2 scales : within a village , examining whether connections within these networks clustered or were uniformly spread ; and among villages , looking at whether food-sharing networks differed according to the village 's remoteness from a population center . Methods : We surveyed 2129 individuals aged 13 years and older in 2003–2004 , within a representative ( block-r and omized ) sample of 21 rural villages in Esmeraldas province , northern coastal Ecuador . We calculated degree ( number of social contacts ) for a social network defined by sharing food . Results : Networks of households sharing food differ according to remoteness from a metropolitan center . On average , residents living in “ far villages ” had 2 more social contacts than those in “ close villages , ” and 12 more years of residence in their village . Estimates of transmissibility ( a measure of outbreak potential ) based on network structure varied as much as 2-fold across these villages . Conclusions : Food-sharing practice s link particular households in rural villages and have implication s for the spread of food-borne pathogens . The food-sharing  networks in remote rural villages are heterogeneous and clustered , consistent with contemporary theories about disease transmitters . Network-based measures may offer tools for predicting patterns of disease outbreaks , as well as guidance for interventions Human behaviour is thought to spread through face-to-face social networks , but it is difficult to identify social influence effects in observational studies , and it is unknown whether online social networks operate in the same way–. Here we report results from a r and omized controlled trial of  political mobilization messages delivered to 61 million Facebook users during the 2010 US congressional elections . The results show that the messages directly influenced political self-expression , information seeking and real-world voting behaviour of millions of people . Furthermore , the messages not only influenced the users who received them but also the users ’ friends , and friends of friends . The effect of social transmission on real-world voting was greater than the direct effect of the messages themselves , and nearly all the transmission occurred between ‘ close friends ’ who were more likely to have a face-to-face relationship . These results suggest that strong ties are instrumental for spreading both online and real-world behaviour in human social networks
target: Second , we show that network composition , individual network central ity , and network structure are associated with important health behaviors and health and development outcomes in different context s across multiple levels of analysis and across distinct network types .
  Lastly , we highlight the opportunities for health research ers and practitioners in LMICs to 1 ) design effective studies and interventions in LMICs that account for the sociocentric network positions of certain individuals and overall network structure , 2 ) measure the spread of outcomes or intervention externalities , and 3 ) enhance the effectiveness and efficiency of aid based on knowledge of social structure .
background: In low- and middle-income countries ( LMICs ) , naturally occurring social networks may be particularly vital to health outcomes as extended webs of social ties often are the principal source of various re sources .