The importance of local interactions

Agent-based systems operate under the assumption of a population of indepen- dent and autonomous entities making decisions in a distributed manner. These

interactions can be described aslocal in the sense that they are based on a non-

global view of information and cause non-global effects on the environment. In aggregate, local interactions can become system-wide trends, defining the be- haviour of the system. While many MAS can be designed from a top-down perspective, as the complexity and size of the system increase this approach becomes increasingly difficult. Mataric (1993) notes that the top-down view limits exactly the type of interactions that result in complex behaviour in na- ture: “the global behaviour of complex systems [...] is determined by the local interactions of their constituent parts”. As a result, much MAS research focuses on manipulating the choices of individuals with a view to changing the system- wide trends that result. In this thesis, we focus on manipulations designed to engender cooperative or coordinated behaviour. We are primarily concerned with two mechanisms for altering the choices of agents in local interactions:

trust and reputation (Chapter 3) and conventions and norms (Chapters 4, 5, 6, 7). Many other mechanisms exist, such as biasing partner-selection through tags (e.g. Griffiths (2008)), but we have chosen to investigate trust, reputation, norms and conventions since these are (i) major mechanisms for cooperative and coordinated behaviour and (ii) highly complementary: trust and reputa- tion manipulate partner selection and norms and conventions manipulate action selection, meaning that the two classes of mechanism can be used in parallel.

2.2

Trust and reputation

Trust and reputation are highly successful mechanisms for supporting coopera-

tive and coordinated behaviour (Josanget al., 2007; Nowak & Sigmund, 1998;

Pirzada & Mcdonald, 2006; Ramchurn et al., 2005). Trust has been subject

to many attempts at definition and we adopt the definition of Ramchurn et

al. (2005): “Trust is a belief an agent has that the other party will do what it

says it will (being honest and reliable) or reciprocate (being reciprocative for the common good of both), given an opportunity to defect to get higher payoff”.

Trust encourages acts of direct reciprocity, and accordingly requires signif- icant historical interaction data for accurate assessments (which can limit its accuracy and applicability). Instantiations of trust in MAS often make use of

multiple dimensions of information (Huynh et al., 2006; Sabater et al., 2006)

including that of reputation, typically defined as a socially known and accepted assessment of trustworthiness. The reputation component of trust and reputa- tion systems thus allows societies to benefit from indirect reciprocity. Indirect reciprocity has been shown to be a greater force in encouraging cooperative behaviour than direct reciprocity in domains with a low probability of repeat interaction (Bravo & Tamburino, 2008). Given that many open MAS domains display this property, reputation is likely to be far more effective than simple trust mechanisms.

sults also tend to be the most architecturally complex (Huynh et al., 2006;

Sabater et al., 2006). Given the properties of open MAS systems, such mech-

anisms may not be as suitable as those that have less complex requirements. However, while robust systems tend to incorporate more complex architectures, systems have been demonstrated that are remarkably simple and support coop-

erative behaviour in low-overhead environments. For example, image scoring,

initially proposed by Nowak and Sigmund (1998), is a simplified model of rep- utation that exhibits low space and time complexity and promotes cooperation through indirect reciprocity. Similarly, Pirzada and Mcdonald (2006) exploit domain-specific insights to develop a low-overhead trust model applicable to communication routing in ad-hoc networks. The Pirzada and Mcdonald model is highly domain specific, and therefore not applicable to open MAS systems

in general. Image scoring is perhaps the most applicablelightweight reputation

mechanism available, but it is likely to be vulnerable to the challenges of open MAS. We investigate this further in Chapter 3.

Trust and reputation, while useful mechanisms for supporting cooperation, are not universally applicable as a solution to cooperative and coordinated be- haviour in open MAS. They do not perform well in systems in which agents can be anonymous (since it is necessary to be able to link a single individual to an interaction history or reputation assessment), and they do not provide

an account of how agents can coordinate their actions, orwhat action to select.

Norms and conventions are a useful complementary mechanism that account for these limitations.

2.3

Norms and conventions

Conventions are generally thought of as socially accepted expectations of be-

haviour, and represent an aggregation of a population’s choices in its individual

interactions. System designers are typically concerned with reducing the cost associated with malcoordination between agents, and conventions are a useful

abstraction for analysing the behaviour of large numbers of agents, to support this aim.

A wide variety of definitions have been proposed in the literature. Lewis (1969)

defines a convention as aregularityin the behaviour of a population in repeated

iterations of the same situation, subject to constraints such as the proportion of agents that conform to the regularity and the proportion of agents that expect others to conform. Goyal (1997) describes conventions as an arbitrary solution to a social problem, wherein individuals only conform because they expect oth- ers to conform. Shoham and Tennenholtz (1997) approach conventions from a game-theoretic perspective, defining a convention as a restriction of agents’ decisions to a single choice in a given coordination game. Kittock (1993) con- siders a convention to exist when a high proportion of agents use the same given strategy. There is little universal agreement on what constitutes a convention, or conventional behaviour, and the theory of convention emergence is under- developed past the initial emergence phase. In Chapter 4, we propose a new formalism of conventions that unifies the above definitions into a cohesive frame- work that describes what conventional behaviour is and allows investigation into the entire convention lifecycle, which was previously not possible.

It is important to distinguish between conventions andnorms, which also

represent socially-accepted rules governing behaviour, but are generally consid-

ered to include an obligation to act according to the norm. Norms are thus

a stronger form of convention, with mechanisms to encourage norm emergence

typically includingincentivesorsanctionsto motivate agent adherence (Agotnes

et al., 2009; Axelrod, 1986; Modgilet al., 2009; Perreau de Pinnincket al., 2009;

Savarimuthu et al., 2007; Sethi & Somanathan, 2002). Such mechanisms may

require additional agent-level or society-level components, which may not always be practical. How sanctions and incentives can be practically applied remains an open research problem, with typical approaches including ostracism of defecting

agents (e.g. Villatoro et al. (2011)). In this thesis, we focus on the behaviour

although we do investigate lightweight sanctions and incentives in Chapter 7. Conventions and norms can be viewed either as rules that are explicitly reasoned upon using some representative language, or as implicit phenomena that emerge from the choices of agents in repeated interactions. Each char- acterisation involves a number of areas for investigation: (i) using explicitly represented norms, fundamental concerns involve representation, reasoning and

mechanisms for enforcement (e.g. Alechinaet al.(2012), Boella & Torre (2004),

Boella & Tosatto (2012), Modgil et al. (2009)), and (ii) considering implicit

norms, research has focused on how norms emerge dynamically, the impact of

network structure, and questions of enforcement (e.g. Axelrod (1986), Pujol et

al.(2005), Salazaret al.(2010b), Sen & Airiau (2007), Villatoroet al.(2009a)).

In this thesis we are concerned with the second characterisation, which sees social processes such as observation, imitation, and information propagation as key mechanisms by which agents coordinate strategy selection and shared and implicit norms and conventions subsequently emerge.