We conducted an experiment using ECoSoS, a self-developed computer-based system, with 152 subjects who accessed the system using a Web browser (see Methods). Our experiment focused on behavioural differences relating to information disclosure. Subjects played repeated rounds of an ‘indirect helping game’. Using the setup shown in Fig. 1, in each round, a subject could receive information about the actions of a recipient and those of the recipient’s own recipients in the previous five rounds at no cost before the subject decided whether to cooperate with the recipient. Note that all data (five instances of what data and whom data) were made available to subjects before deciding. Subjects could receive at most four items of either what or whom data by pushing the disclosure button on the information disclosure screen.

Figure 1 Disclosable information subjects can receive. This illustration shows the information disclosed to subjects in an ‘indirect helping game’ before they decide on their actions (cooperation or defection) in each round. Every subject meets a recipient, called X, who is assigned by the system in each round. The information about X falls into two categories. First, X’s actions (what data) in the previous five rounds are disclosable. For example, if what data (X’s previous action) in the third-last round is disclosed, either ‘C’ (cooperation) or ‘D’ (defection) is shown. Second, X’s recipients’ actions (whom data) in the previous five rounds are also disclosable. For example, if whom data (about X’s recipient) in the third-last round is disclosed, an integer between 0 and 5 is shown. The integer represents how many times X’s recipient cooperated in the five rounds preceding that round (the number of ‘C’ values), in this case, the five rounds from the fourth-last round to the eighth-last round. Therefore, the total number of pieces of disclosable information about X is ten (five items of what data and five items of whom data). Full size image

Information disclosure behaviour

Our analysis of the results of the experiment shows that information disclosure behaviour exhibits several features (Fig. 2). First, the top four situations in terms of the number of disclosures are as follows: two items of what data and two items of whom data (38%), four items of what data (20%), three items of what data and one item of whom data (14%) and no items (14%) (Fig. 2a). To determine the relationship between disclosed what data and disclosed whom data, Fig. 2b shows the non-negligible number of cases of whom data priority (15%; defined in the caption of Fig. 2) as well as those of what data priority (25%), while those in which only what data are disclosed (25%) are the most frequent.

Figure 2 Information disclosure behaviour. The graphs show the disclosure behaviour of all subjects (N = 152) except for the first ten rounds and the 52nd and later rounds (# = 6,232). (a) The number of disclosures in the case of (x, y) in each round where x and y represent the number of disclosures of what data and whom data, respectively. (b) The number of disclosure types. These were counted for each subject in each round. To define what data priority, we introduce the concept of a data pair and a what-prior pair. A data pair consists of what data and its corresponding whom data a data pair. A what-prior pair is a data pair in which its what data was disclosed before its whom data (including the case where the whom data was not disclosed). If all five data pairs in a round are what-prior pairs, this set of pairs is said to have what data priority. Whom data priority is defined as having all five data pairs in a round as whom-prior pairs (whom data is disclosed before the what data). (c) The disclosure rates of pairs (what data and whom data in the same round) and the Fisher’s exact test values for all pairs. For example, “DISCLOSING ‘C’” is the disclosure rate of whom data when the previously disclosed what data for the pairs were ‘C’. The Fisher’s exact test values show that the content of what data has no statistically significant effect on disclosed behaviour in relation to whom data, while the content of whom data has a statistically significant effect on disclosed behaviour in relation to what data. Statistics are shown in Table S1 in Supplementary Information. Full size image

Most studies on reputation-based cooperation refer to what data and whom data as first-order and second-order information, respectively4. These terms imply that second-order information is only disclosed after first-order information has been disclosed. However, our experiment showed that players observe what data and whom data independently. Our results show a case in which first-order information is disclosed in accordance with the content of the second-order information that was previously disclosed.

The second feature of information disclosure behaviour is that it depends on the content of previous disclosures (Fig. 2c). In this study, the labels ‘G’ (good) and ‘B’ (bad) correspond to whom data values of ‘3 or more’ and ‘2 or less’ ‘C’ (cooperation) actions, respectively, in the previous five rounds. The Fisher’s exact test results for all subjects show that the disclosure rate for what data when whom data that was previously disclosed was ‘G’ is significantly higher than the disclosure for that when whom data that was previously disclosed was ‘B’ (P value = 0.0007029, one-tailed test). In contrast, there was no significant difference in the disclosure rate of whom data when what data that was previously disclosed was either ‘C’ or ‘D’ (P = 0.1384).

Theoretically, there is no reason for players not to receive what data even though the whom data in the pair had previously been disclosed. Thus, as shown in Fig. 2c, more than 60% of what data were disclosed when the whom data in the pair had previously been disclosed, while less than 30% of whom data were disclosed when the what data in the pair had previously been disclosed. Fisher’s exact test values in relation to the cases of ‘G’ and those of ‘B’ suggest that players tended not to care about information when whom data were ‘B’. To determine if this evidence of selective inattention, we needed to perform a statistical analysis of cooperative actions.

Cooperation and information disclosure

We tested whether information disclosure behaviour influences decision-making. To do this, we categorized each round of each subject into seven models and applied those with logistic generalized linear mixed models, as shown in Table 1. Model 1 (with no disclosure) shows that decisions regarding cooperation are positively influenced by three factors (subjects’ current total earnings, the decisions by subjects in the previous two rounds and whether they previously received help as a recipient) and negatively influenced by one factor (the round number), as found by many previous studies15,26.

Table 1 Determinants of cooperation. Full size table

Our experiment supports the proposition that what data and whom data play completely different roles in terms of cooperative behaviour. Model 2 shows that subjects reacted to what data in a strongly reciprocal way; they cooperated with others who had cooperated previously, consistent with the findings of Swakman et al.10. Such ‘indirect reciprocity’ is widely observed and repeatedly studied. In contrast, Model 3 shows that whom data had little influence on decision-making when subjects received only whom data. However, this is a rare situation because those included in Model 3 only comprise 3.0% of all cases, while the number comprising Model 2 is about eight times greater (25.0%). This is theoretically true because the whom data have no relationship with the recipients themselves, and thus disclosing whom data without disclosing what data is entirely irrational. Subjects may be confused by the situations. Model 3 has no significant factors except for previous decisions by the subjects themselves.

We tested the aggregate effects of what data and whom data in terms of decision-making regarding cooperation by comparing Models 4 to 7. First, we considered the cases of what-prior pairs (Models 4 and 5). When the what data of what-prior pairs were ‘D’, the whom data of the pairs (either ‘G’ or ‘B’) had a strong effect on decision-making (Model 5). However, the results of Model 4 showed that the content of whom data had no effect on decision-making when the what data of what-prior pairs were ‘C’. This is inconsistent with Swakman et al.’s findings (Model 3 in their paper), and thus we need to conduct further experiments.

Comparing Models 4 and 5, attention by the players depends on the content of what-prior data. On the one hand, if the what data are ‘C’, neither what data nor whom data have a significant influence on their decision-making. Helping others who cooperated in the previous round corresponds to the mechanism of ‘indirect reciprocity,’ whereby the player helps the recipient because they helped someone else. On the other hand, if the what data are ‘D’, both what data and whom data have a significant influence on their decision-making. Subjects check whether or not the recipients’ previous defections are justified. ‘Justified defection’17 has been repeatedly considered by many theoretical and empirical papers.

Second, we consider the case of whom-prior pairs. Unprecedented insights are revealed by a comparison of Models 6 and 7. When the whom data in whom-prior pairs were ‘G’, the what data of the pairs (either ‘C’ or ‘D’) had a strong effect on decision-making (Model 6). If recipients cooperated with good players in the previous round, subjects chose cooperation. This ‘prosociality’27,28 or ‘prosocial chain’ is regarded as an extended version of ‘indirect reciprocity’, that is, a player helps a recipient because they helped good people, or a player refuses to help a recipient because they refused to help good people.

In contrast to Model 6, Model 7 shows that when the whom data in whom-prior pairs were ‘B’, the what data in the pairs had no significant influence on reputation-based cooperation. Even if recipients had played with bad players in the previous round, subjects did not care about this information. This is evidence of selective inattention playing a role in reputation-based cooperation.

Selective inattention

In most studies on the norms of reputation-based cooperation, it has been debatable how people assess donors’ actions when the donors play with those who have bad reputations. For example, Kandori (1992)29,30 applied a rigorous rule, later termed stern-judging, whereby cooperation with bad players is assessed as bad and defection to bad players is assessed as good. Sugden19 proposed a more tolerant rule, later termed simple-standing, as mentioned earlier. These rules have been compared numerous times, both theoretically12,31 and empirically16. The results of our study suggest that the possibility of selective inattention should be a main focus. The data presented in Table 1 suggest that subjects intentionally ignore information regarding reputations in specific situations. In particular, Model 7 indicates the existence of selective inattention, whereby subjects do not consider the previous actions of those who have bad reputations. This empirical fact is entirely consistent with the staying norm24,25,32 because this norm ignores recipients’ actions and maintains their reputations even when their previous recipients had bad reputations.

As shown in Fig. 3, subjects’ cooperative actions depend on the content of the information about their recipients. When the recipients’ previous actions were ‘C’, the subjects cooperated with them without considering any other information. In contrast, if the recipients’ previous actions were ‘D’, the subjects tended to check whether their refusal to cooperate was justified. In addition, if the recipients’ previous recipients were ‘G’, the subjects tended to check whether their recipients engaged in prosocial action, i.e., cooperation.

Figure 3 Flowchart of decision-making using information about recipients. This conceptual flowchart represents a summary of the factors determining cooperative behavior in Models 4 to 7 in Table 1, where the labels ‘G’ and ‘B’ correspond to ‘3 or more’ and ‘2 or less’ C actions, respectively, in the previous five rounds. When a subject first receives what data about their recipient, they obtain either ‘C’ or ‘D’ as the recipient’s action. If ‘C’, the subject decides to cooperate with the recipient regardless of the content of the whom data. If ‘D’, the subject then receives the related whom data, in which they obtain either ‘G’ or ‘B’ regarding the recipient’s previous recipient. The subject cooperates with of defects against the recipient if the whom data is ‘G’ or ‘B’, respectively. When the subject first receives whom data about the recipient, they obtain either ‘G’ or ‘B’ regarding the recipient’s previous recipient. If ‘B’, the subject does not use this information to determine their action, but considers other information (decisions they made in the previous two rounds) to decide their action. If ‘G’, the subject receives the related what data. The subject cooperates with or defects against the recipient if the what data is ‘C’ or ‘D’, respectively. Full size image

Our results highlight the importance of focusing on the what data or first-order information. Subjects cooperated with those who had previously cooperated. This is known as downward indirect reciprocity8. This prosociality (cooperation to cooperation) reflects a chain of prosociality (cooperation to cooperation to cooperation), which is supported by Model 6 in Table 1. Moreover, our results support the proposition that people consider so-called justified defection.

Surprisingly, if the recipients’ previous recipients were ‘B’, the subjects did not consider the information regarding the recipients, and based their decisions on other information. The subjects looked at the information about their recipients but did not consider it. This selective inattention is supported by the Fisher’s exact test values in relation to the information disclosing behaviour shown in Fig. 2c.

Although economic models are usually based on the assumption that agents are unconstrained in their ability to process information, economists have long recognized that individuals have limited cognitive abilities33. A series of empirical studies on these limited abilities has repeatedly shown that the human trait of rational inattention34 is used as a shortcut for complex information processing. For example, Lactera et al.22 found left-digit bias (a tendency to focus on the leftmost digit of a number while partially ignoring other digits) in the processing of odometer values after analysing more than 22 million wholesale used-car transactions.

Several studies have observed not only defensive inattention in relation to complex information processing, but also proactive inattention. Cheremukhin et al.’s experimental evidence23 strongly suggests the conscious disregard of information rather than limitations in a decision-maker’s cognitive abilities. The majority of errors by subjects in their experiment arose from deliberate decisions to ignore some of the available information.