Acculturation refers to phenomena which result when groups of individuals having different cultures come into continuous first-hand contact with subsequent changes in the original cultural patterns of either or both groups.

Gordon’s unidimensional acculturation model states that acculturation is a process of change in the direction of the mainstream culture. The outcome is adaptation to the mainstream culture. Unidimensional models are controversial as migrants more often desire to have a bicultural identity due to the increasing magnitude of migration and a change in the view of migrants (i.e. more accepting climate).

Berry’s bidimensional acculturation model states that the degree of acculturation depends on the degree of adaptation and the degree of cultural maintenance. This leads to several strategies:

1. Integration (i.e. biculturalism)
   This refers to the combination of both cultures and has the preference of migrants.
2. Separation
   This refers to maintaining the original culture and not adapting to the host culture.
3. Assimilation
   This refers to complete absorption of the host culture with loss of the original culture.
4. Marginalisation
   This refers to the loss of the original culture without adapting to the host culture (i.e. negative view of both cultures).

It is important to take acculturation into account with assessment as it is necessary to know whether the person is part of the population on which an instrument has been tested and whether the instrument can measure the intended construct with that person (e.g. may not be possible due to implicit assumptions of the instrument about general knowledge). Acculturation can be a moderator of test performance.

A culture refers to a largely shared, unchanging and internalised sets of beliefs, values and practices which are transmitted across generations and which constrain human behaviour in context. Individuals differ in their level and strategy of acculturation. To measure acculturation, it may be useful to measure maintenance and adaptation dimensions (1), contact, change and identity aspects (2), domain specificity (3) and psychological and sociocultural outcomes (4).

Acculturation measures allow for different combinations of positive or negative attitudes towards adaptation (i.e. adjusting to the host culture) and maintenance (i.e. holding on to the original culture). There are three question formats to measure this:

1. One-question format (e.g. cultural integration-separation index; CIS)
   This includes a forced choice between valuing the ethnic culture, the host culture, both cultures or neither.
2. Two-question format (e.g. acculturation in context measure; ACM)
   This asks for separate importance rating for adapting and maintaining the culture.
3. Four-question format (e.g. acculturation attitudes scale; AAS)
   This requests agreement ratings for questions representing all acculturation strategies.

The two- and four-question format appear to be most successful in discriminating between strategies. Hutnik proposed four identity strategies:

1. Acculturative identity (i.e. hyphenated identity)
2. Assimilative identity (i.e. predominantly host culture identity)
3. Dissociative identity (i.e. embedded minority identity)
4. Marginal identity (i.e. indifferent to minority and majority identity)

It may be useful to assess acculturation identity when it comes to assessment by using the twenty statements test (1), self-categorization test (2), ethnic identification scale (3) or the multigroup ethnic identity measure (4). Resistance to acculturation appears to be most persistent when it comes to identity aspects.

A contextual acculturation model states that acculturation orientations are simultaneously influenced by pressure by migrant’s ethnic community and the host society. The preferred acculturation strategy may differ per domain (i.e. public domain or private domain). This can be measured by using the ACM (i.e. acculturation in context measure) asking questions about the home (1), family (2), school (3) and work situations (4). Migrants often value maintenance more in private domains than in public domains. Indicators of acculturation should be a balanced sample of behaviours and attitudes on both sides of the public-private domain divide.

Two possible outcomes of acculturation are psychological adjustment and sociocultural adaptation. Psychological adjustment includes subjective well-being (1), satisfaction self-esteem (2) and psychological health (3). Sociocultural adaptation involves the acquisition of effective behaviours (1), social skills (2), language mastery (3) and cultural knowledge (4). High levels of social support (1), low incidence of life changes (2) and an internal locus of control (3) predict psychological adjustment. A longer period of residence (1), a smaller cultural distance at group level (2), more frequent interactions with host nationals (3), more favourable attitudes toward cross-cultural adaptation (4), more emotional stability (5) and extraversion (6) predict sociocultural adaption. It may be necessary to measure both outcomes to assess acculturation through measuring acculturation stress (e.g. by using the satisfaction with life scale).

An item or instrument is biased if it does not have the same meaning across the groups studied. There are three typed of bias:

1. Construct bias
   This refers to an incomplete identity of a construct across groups or incomplete overlap of behaviours associated with the construct (i.e. a construct cannot be used in both groups). Prevention of this bias requires extensive knowledge of the culture of interest.
2. Method bias
   This refers to bias resulting from the methodology (e.g. instrument characteristics). This can be prevented by using a detailed protocol (1), extensive training (2) and detailed instructions (3).
3. Item bias
   This refers to anomalies at item level. It means that people with the same standing on the underlying construct do not have the same average score on the item (e.g. migrants and host culture). This can be prevented using psychometric techniques.

There are three reasons to include measures of acculturation in multicultural assessment:

1. It allows to identify problems in the acculturation process.
2. Acculturation has a pervasive influence on behaviour and knowledge on this could map this influence better.
3. It allows for detection of acculturation-based biases in psychological tests.

One problem in the assessment of multicultural groups is the dependence of the outcomes on the level of adjustment. There are several methods of approaching the potential cultural bias:

1. Use cut-off scores on an acculturation instrument
   This means that a cut-off score is used to determine when an instrument can or cannot be interpreted in the standard way.
2. Use hard acculturation data to establish differential norms
   This means that acculturation data (e.g. length of stay in host country) are used to establish norms for when an instrument can or cannot be interpreted in the standard way (e.g. use corrected scores).
3. Use acculturation as a covariate or moderator
   This means that it should be tested how deviant a tested person is from the standardization sample using acculturation (i.e. correlation scores of the acculturation instrument with the scores on the target instrument to correct a score).
4. Use standardization or centring
   This refers to takin the deviation scores from the individual or group mean to eliminate group differences due to response styles.
5. Use the item response theory
   This holds that when items of an instrument meet the assumptions of item response theory among both migrants and hosts, the instrument can be used in both groups.
6. Use method factors
   This holds that a complex statistical method should be employed to balance our response tendencies.
7. Use the person-fit tradition
   This states that expectations about the score patterns of migrants can be formulated using common score patterns of mainstream participants. This allows to determine the extent to which it is fair to assume that a particular migrant psychologically becomes to the mainstream group.