Chapter 2 of Interpersonal Perception: The Foundation of Social Relationships and Table 5.1 on page 119 and 5.2 on page and 123.
Assimilation reflects the extent to which a perceiver rates targets in the same way; using the Social Relations Model, assimilation is assessed by the degree of perceiver variance and is typically measured as the proportion of the total variance due to the perceiver. This proportion can be interpreted as the correlation of two ratings by the same perceiver of two different targets. Using Laing notation, it is symbolized as A(B) = A(C).
The question concerns whether the perceiver sees the targets the same. More colloquially does the perceiver think that the "targets all look alike"? Note that if all of the perceivers see the targets in the same way, there is not assimilation. Assimilation requires that one perceiver see the targets in one way and another perceiver see the targets in another way. Finally, note that assimilation refers to the targets also. So if the judgments are same-gendered, the degree of assimilation refers to same-gendered partners. Different results might be obtained the partners were members of the other gender.
The average level of assimilation is about .25 in judgments of the Big Five which means that the perceiver effect explains about twenty-five percent of the variation in the perception of others. Alternatively it can be viewed as the correlation between a perceiver's ratings of two targets is .25. For liking, the level of assimilation is a typically somewhat higher.
Assimilation is less for more observable traits (e.g., Extroversion), especially when they are highly evaluative (Kenny & West, 2010).
Assimilation appears to decline with increasing familiarity, which is consistent with view that the perceiver effect represents a guess of what others are like. For first impressions, the percentage of perceiver variance is .32, it declines to .28 after spending a bit of time with the target, and is .22 when the perceiver is very familiar with the target. For liking judgments, assimilation may in fact increase due to changes in how much the person likes being in the group.
Assimilation tends to be greater for judgment of members of out-groups than it is for in-groups (Boldry & Kashy, 1999). Assimilation is greater in computer-mediated interactions and in groups more than one-on-one interactions.
There is evidence that the perceiver effect is related to gender (Winquist, Mohr, & Kenny, 1998). Also it has been shown the perceiver effects are greater those who are securely attached, not depressed, and not narcissistic. All of these effects are rather small, correlations of no more than .20.
Both Wood and colleagues (2010) and Srivastava and colleagues (2010) have found that the perceiver effect is strongly correlated across the Big Five. Most, but not all of that covariation is determined by positivity: Some perceivers generally see targets positively and others generally see them negatively. In part, this reflects more of tendency to like members of the particular group being judged, not a general tendency to like members of all groups. Moreover, there are some reasons to believe part of the positivity effect reflects one's transient mood.
Once positivity is removed, there is what has been called trait-specific perceiver effects, what was called personal stereotype in the Interpersonal Perception books. Rau and colleagues (2021) investigated trait differences in how much of their variance is due to positivity and how much is due to trait-specific variance. They found that perceiver effects were more due to trait-specific effect for traits higher in observability, lower in evaluativeness, and in contexts with less personal involvement but more familiarity.
Srivastava and colleagues (2010) and Rau and colleagues (2021) also find evidence for a relatively weak acquiescence effect or the tendency to give high (or low ratings) to all targets.
Perceiver effects tend to be fairly stable over time. Several studies show increasing consistency over time and Wood and colleagues found a one-year stability of .69. For shorter intervals once perceivers get to know the targets the stabilities can be as high as .80. However, the evidence, as reviewed by Kenny (1994), judgments made at zero acquaintance show much lower levels of stability of .64 and attraction judgments show lower stability.
Here the question concerns whether the perceiver effect for one group of targets (e.g., friends) is the same as the perceiver effect for a different group of targets (e.g., co-workers) The perceiver effects for in-group and out-group members are positively correlated, but the correlation is relatively weak for liking judgments. Data from Malloy and colleagues (1997) finds consistency in the perception of family members, co-workers, and friends, correlations of about .51. Finally, in perhaps one of the strangest interpersonal perception study ever conducted, Kwan, Gosling, and John (2006) find that the perceiver effect is correlated across the perception of other people and dogs, r = .56. People and dogs are seen as more similar than different types of people.
Assimilation is a key topic in the discussion of Assumed Reciprocity and Assumed Similarity.
Boldry, J. G., & Kashy, D. A. (1999). Intergroup perception in naturally occurring groups of differential status: A social relations perspective. Journal of Personality and Social Psychology, 77, 1200-1212.
Kenny, D. A., & West, T. V. (2010). Similarity and agreement in self- and other perception: A meta-analysis. Personality and Social Psychology Review, 14, 196-213.
Malloy, T. E., Albright, L., Kenny, D. A., & Agatstein, F. (1997). Interpersonal perception and metaperception in nonoverlapping social groups. Journal of Personality and Social Psychology, 72, 390-398.
Kwan, S. Y., Gosling, S. D., & John, O. P. (2006). Anthropomorphic projections of personality: A cross-species Social Relations Model Analysis of humans and dogs. Unpublished paper, Arizona State.
Rau, R., Carlson, E. N., Back, M. D., Barranti, M., Gebauer, J. E., Human, L. J., Leising, D., & Nestler, S. (2021). What is the structure of perceiver effects? On the importance of global positivity and trait-specificity across personality domains and judgment contexts. Journal of Personality and Social Psychology, 120, 745–764.
Srivastava, S., Guglielmo, S., & Beer, J. S. (2010). Perceiving others’ personalities: Examining the dimensionality, assumed similarity to the self, and stability of perceiver effects. Journal of Personality and Social Psychology, 98, 520-534.
Winquist, L. A., Mohr, C. D., & Kenny, D. A. (1998). The female positivity effect in the perception of others. Journal of Research of Personality, 32, 370-388.
Wood, D., Harms, P., & Vazire, S. (2010). Perceiver effects as projective tests: What your perceptions of others say about you. Journal of Personality and Social Psychology, 99, 174–190.