According to Funder (1999), good information has two facets—quantity and quality. One feature of the quantity of information a judge can use pertains to his or her level of acquaintanceship with the target. However, even so-called "zero-acquaintance" studies usually yield substantial informants' consensus regarding basic personality traits (Borkenau & Liebler, 1992; Chaplin, Phillips, Brown, Clanton, & Stein, 2000; Kenny et al., 1994). There is also evidence that personality ratings by strangers resulting from enough "thin slices" of (sometimes video-
based) behavior can reach high consensus and substantial accuracy, which can be additionally increased if ratings of multiple informants are combined (e.g., Ambady & Rosenthal, 1992; Borkenau et al., 2001, 2004; Borkenau & Liebler, 1993; Kenny, 1994; Levesque & Kenny, 1993; Watson, 1989).
The level of acquaintanceship, however, is difficult to quantify because it is related to time and contexts in which informants and targets have been together. In general, the validity of informant assessment benefits from the acquaintanceship effect. A simple explanation for this effect is that increased length of acquaintance is probably accompanied by more information (e.g., Bernieri et al., 1994; Blackman & Funder, 1998; Colvin & Funder, 1991; Funder & Colvin, 1988; Funder, Kolar, & Blackman, 1995; Kurtz & Sherker, 2003; Paulhus & Bruce, 1992; Paunonen, 1989; Stinson & Ickes, 1992; Watson & Clark, 1991; Watson et al., 2000). One question about the acquaintanceship effect is, however, concerned with the possibility of assumed similarity, reflecting that well-acquainted judges resemble their targets and achieve accuracy by simply projecting their self-concept on them. Some studies have tried to rule out assumed similarity (e.g., Funder et al., 1995; Watson et al., 2000), whereas others have asserted that where informants and targets are really similar, the use of projection constitutes a successful and reasonable heuristic, instead of just an artifact, to achieve accurate judgments (Kenny & Acitelli, 2001; Neyer, Banse, & Asendorpf, 1999).
Self-other agreement and consensus may be quite differently affected by the acquaintanceship effect. From the WAM perspective, Kenny (1994) anticipated that consensus would be established very early when judges share stereotypes. Over time, however, these stereotypic judgments would be replaced by judgments deduced from actual behavioral observation leading to a change in the content of consensus rather than its level, although at the same time, accuracy in terms of behavior prediction would be improved. This exact process was observed in an experimental study by Blackman and Funder (1998). However, even when accuracy increases, accuracy cannot exceed consensus for the same basic psychometric reasons that validity cannot exceed the square root of the reliability.
Finally, the acquaintanceship effect may also be context specific (Branje, van Aken, van Lieshout, & Mathijssen, 2003; Kurtz & Sherker, 2003). Informants may know the targets from different contexts (e.g., school, workplace, marriage, family, etc.). Achenbach et al. (1987) reported high agreement between the mother and father's ratings and between teachers' ratings of behavioral problems of schoolchildren, but a much lower agreement was found between the parental and teacher ratings. Although the parental and teacher judgments were not independent, because they may have talked to each other about the children, they certainly based their judgments on different contexts. Thus, although informants may have varying opportunities to observe the target in different situations, they may be equally accurate in predicting how the target will behave in other situations. Funder (1999) argued that this could be explained by the ability of the human judge to generalize his or her judgments from one context to a vastly different context.
Whereas the acquaintanceship effect pertains to the sheer amount of information that informants share about targets, the quality of information is also important. The issue of information quality is related to the question of where it is best to look for certain traits. Almost two decades ago, Anderson (1984) showed that listening to people talk about their thoughts and feelings results in a more accurate personality judgment than listening to people talk about their hobbies and leisure activities. More recently, some studies have addressed this question more profoundly. A study on handshaking and first impressions, Chaplin et al. (2000), showed that firm handshakes were related to extraversion, emotional stability, and openness to experience (the latter was only true for women). Gosling et al. (2002) showed in a study that personality judgments when viewing offices and bedrooms were consensual between independent observers and could predict self-rated personality traits (e.g., conscientiousness and openness to experience). Moreover, Gosling et al. found that both environmentally based consensus and accuracy were comparable and sometimes even stronger than the levels in zero-acquaintance and long-term acquaintance studies, as was summarized by Kenny (1994). It therefore seems that personal environments contain richer information for informant assessment than zero-acquaintance contexts, and sometimes even more than long-term acquaintance contexts.
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