To control outbreaks of (emerging) diseases at an early stage, effective collaboration between human and veterinary healthcare professionals is essential . However, to date, collaboration has taken place only on a very limited scale [1,2,3,4,5,6]. For this reason, One Health, an interdisciplinary approach addressing the connections between health care for humans, animals and the environment and focused on the elements biomedical research, enhanced public health efficacy, an expanded scientific knowledge base and improved medical educational and clinical care in which human and veterinary healthcare and other stakeholders work together , is placed high on the agendas of organizations such as the WHO, the European Commission’s Directorate-General for Health and Consumers, USA Centers for Disease Control and Prevention [CDCP] and Worldbank [1, 8,9,10].
Because more than three-quarters of all the infections seen in humans originate in animals , there is great and widespread interest in the effective collaboration between human and veterinary healthcare professionals as a means to halt outbreaks of infections at an early stage. The outbreak of Severe Acute Respiratory Syndrome (SARS), for instance, which was not even known to be an animal-transmitted infection when it first occurred, is estimated to have caused a loss of as much as US$40 billion in terms of Gross Domestic Product (GDP) worldwide . Other examples include the recent outbreaks of avian influenza and Q fever in the Netherlands, which have shown that serious outbreaks can have major consequences for human and animal health . Areas where close cooperation would be fruitful because they share the same goals include combatting and controlling zoonoses and emerging zoonoses; here, the interdependence of the two fields requires an integrated approach. In efforts to reach mutual goals, an interdisciplinary exchange of knowledge between the two professional groups can lead to a sharing of domain-specific expertise and experience, thus enabling experts to identify possible zoonotic infections and to fight them with the help of appropriate control measures at earlier stages . Another advantage of collaboration between these two groups of healthcare professionals, besides the more adequate fight against infectious diseases, is the exchange of knowledge on the treatment of diseases  leading to major healthcare cost savings [6, 11] and to new scientific insights . In order to better understand the mental motivation of health care professionals with respect to the One Health approach, this study used psychosocial concepts such as social dilemma, (group) identification and category thinking.
Collaboration as a social dilemma
Collaboration can be defined as “any action which is intended to benefit others, regardless of whether the actor also benefits in the process” . However, collaboration can also fail to take place: in human and veterinary healthcare, cooperation between professionals can fail to materialize because there is a social dilemma, for instance. To illustrate, there are situations in which a non-cooperative course of action is (at times) tempting for each individual in that it yields superior (often short-term) outcomes for the individual himself or herself . For example, it is commonly accepted that professionals have a constant and permanent responsibility for their patients, who expect care and action in the short term. A healthcare professional may therefore view collaboration, which may (at times) exceed or influence his own day-to-day operations, as not sufficiently important, at least for the short term. But if everyone pursues this non-cooperative course of action, then everyone will ultimately be worse off (often in the longer term) than would have been the case if everyone had collaborated.
Collaboration between human and veterinary healthcare professionals can be characterized as a social dilemma. It is often thought that a lack of time is the main reason why healthcare professionals feel they have so little psychological room for greater collaboration. Fleuren, Wieferink and Paulussen , however, show that healthcare professionals actually have other reasons than time; what lies at the heart of their limited cooperation is a lack of clarity as to why collaboration should be organized in the first place. The individuals concerned do not always have a sufficiently well-defined idea of the benefits offered by increased contact and collaboration, even though collaboration is highly desirable both for the sector itself and for society as a whole . In this regard and in any event, the (long-term) awareness of sharing a common goal is of crucial importance.
Collaboration is stimulated if the (perceived) benefits outweigh the individual arguments or circumstances of the healthcare professionals concerned. This type of calculation, or dilemma, can lead to cooperation ‘for the public good’. Considering its objectives, One Health can be seen as a public good dilemma, because “public good dilemmas require individuals to make an active contribution to establish or maintain a collective good, such as building a local bridge or joining a social movement” . However, in terms of making room for the bigger collective goal alongside their responsibilities related to the day-to-day care of their own patients, human and veterinary healthcare professionals often see insufficient added value , even though a greater awareness of the added value associated with collaboration would ultimately result in improved care [5, 15, 16].
Greater awareness of the added value brought about by collaboration on the part of healthcare professionals is consistent with the social desire for more intensive collaboration between human and veterinary healthcare [1, 8,9,10, 12]. These two groups of healthcare professionals speak the same ‘language’ and should therefore be able to understand each other well . Despite the fact that, broadly speaking, both groups followed similar training programmes [18, 19] and perform similar clinical procedures, there is hardly any exchange of knowledge and experience . In addition, a clear focus on common goals is lacking, and collaboration between human and veterinary healthcare professionals remains limited at present [5, 6, 20]. So far, collaboration has only taken place in a limited number of research areas and only occasionally during outbreaks of emerging diseases [1, 4]. Closer examinations have revealed that the limited scale of collaboration is due, among other things, to mutual prejudices  and psychological barriers between the parties concerned . Such mutual judgements and prejudices may disrupt the development of collaboration, but research into these phenomena has so far remained very limited. The present study is aimed to help fill that gap.
Common ingroup identity theory
In order to gain a better understanding of what stimulates cooperation among healthcare professionals, we used the Common Ingroup Identity Model developed by Gaertner and Dovidio . This theory provides insight into the relationship between individual perceptions and behaviours towards groups and lists possible causes which may influence these. According to the Common Ingroup Identity Model, sharing a common goal affects the degree of collaboration, but this relationship is also influenced by the perception of this degree and the means of categorization [17, 21]. The Common Ingroup Identity Model  focuses on the individual’s perception with regard to the group. In essence, the model states that members who see themselves as belonging to a larger, common whole consciously classify themselves within that larger whole, as a result of which prejudices between groups or communities decrease.
According to Gaertner and Dovidio, collaboration is influenced by characteristics (in this study qualified as the common goal) which play a role in the individual perception of the situation: does the individual perceive the existence of a single overarching group, several groups or subgroups, or no group at all? If individuals feel that they belong to a group, in this case ‘healthcare’, this will lead to more positive thoughts, feelings and behaviours among the individuals in the groups concerned . As explained above, when group members see themselves as part of a larger whole and when they classify themselves within this structure, prejudice between groups or communities is reduced. According to Kramer and Brewer , people within a group or an overarching whole are prepared to share communal resources and other supplies, but they will develop resistance if these have to be shared with others outside the group. Still, a change occurs if these outsiders can be placed within a perceived larger whole. The realization that there is in fact a larger whole means that the positive thoughts, feelings and behaviours (such as the sharing of resources and information) which would normally be reserved for the individual’s own familiar group are extended to members of other communities who also belong to the larger overarching whole. Common endeavour and classifying thus go hand in hand, leading to feelings of ‘us’ rather than feelings of ‘us versus them’. In other words, it depends on how an individual sees his group or subgroup within a larger whole .
Using the Common Ingroup Identity Model, we quantitatively assessed healthcare group interrelationships in order to gain insight into the contributions towards cooperation that can be made by means of a common goal for healthcare professionals formulated via their perception of group formation. In the past, the Common Ingroup Identity Model was researched primarily in experimental studies, with a main focus on perceived group formation . A limitation of the model is that the duration of the effect of an intervention is unclear, as is the reduction of bias . The current study will not only indicate whether the Common Ingroup Identity Model is useful for the respective groups of healthcare professionals, but it will also quantitatively assess the relationships between the common goal and collaboration in combination with associated mediating factors. In this way, the study will contribute to further theoretical development in terms of validation as well as to the quantitative usefulness of the Common Ingroup Identity Model. It will also examine whether the exchange of knowledge is an additional trigger for collaboration once healthcare professionals have become aware of the common goal.
Collaboration and common goal
In the social dilemma referred to earlier, where there is insufficient awareness of the possible advantages of collaboration, and in this case human and veterinary collaboration, crucial factors include the reasons why healthcare professionals place themselves in a particular category and identify with their ‘own’ professional group [2, 3, 5]. It is ‘natural’ for people to engage in social categorization: the brain is hard-wired to think in terms of categories, and categories form the basis for standard judgements and prejudices . The advantages of social categorization are that individuals know where they stand and what is expected of them, and that a group or community contributes to a feeling of (social) well-being . The same is true of human and veterinary healthcare professionals, although a certain distance is maintained between them [2, 5]. After all, what is qualified by the terms ‘human’ and ‘animal’ is placed in different categories. This is illustrated by the dichotomy between the two professional groups, established on the basis of typical activities, with ‘human patient’ being contrasted with ‘animal patient’, and hence on the basis of category-based thinking . Still, mutual contact alone does not lead to productivity or better joint results; for good results, interdependence is necessary [26, 27]. If individuals perceive a common goal, in this case ‘improving care through One Health’, then according to the Common Ingroup Identity Theory this can be expected to lead to more positive thoughts, feelings and behaviours between individuals in the groups concerned , because there will then be more perceived interdependence between the two groups. According to the Common Ingroup Identity Theory, the awareness of a larger whole or a common ingroup, in this case a joint responsibility for improving care, will lead to the extension of positive thoughts, feelings and behaviours (such as the sharing of resources and information) that were traditionally reserved for the individual’s own familiar group to members of other communities who also belong to the larger overarching whole.
In the case of common goals and interests, a clear interdependence can be seen: after all, the aim is to achieve a result which requires contributions from both groups. In their model, Gaertner and Dovidio  describe a common goal in terms of ‘interdependence’. In this respect, collaboration between human and veterinary healthcare professionals is the result of addressing common goals and interests [2, 3, 6, 11, 16]. This is in line with the social interdependence theory which argues that interdependence results from a common goal [27,28,29,30,31]. Mutatis mutandis, this altruistic goal was recently incorporated in the One Health initiative (16). This means that One Health, as a common goal, can be expected to lead to greater collaboration. This brings us to our first hypothesis:
One Health as a common goal has a positive effect on collaboration between human and veterinary healthcare professionals.
The process of classifying concerns the individual’s perception of the connection between – in this case – two groups of healthcare professionals: how an individual sees his group or subgroup within a larger whole and whether the individual perceives the existence of a single overarching group, multiple groups or subgroups, or no group at all . As described earlier, One Health can affect how people see themselves as part of a larger, common whole and how they classify themselves within that larger whole. The Common Ingroup Identity theory distinguishes the following types of perception and reclassification: recategorization, decategorization and mutual differentiation . These types are elaborated below.
Perceived commonality and perceived common goals (overlap between groups and a stronger feeling of ‘us’ rather than ‘us and them’, recategorization) result in greater collaboration [22, 32]. That being said, the feeling of ‘us’ is not by definition limited to a single group: a person can possess multiple identities because he or she can be a member of multiple groups . This means that in addition to classifying themselves in the veterinarians’ group, veterinarians could also classify themselves in the (overarching) group of healthcare providers . We speak of recategorization when an overarching identity is perceived in which old groups are represented as a whole or in a new form, for example as a subgroup. Via the formation of a subgroup, collaboration between the two groups of healthcare professionals is further enhanced, for instance through awareness of a common goal. This brings us to our second hypothesis:
The positive relation between common goal and collaboration is mediated by the partial effect of recategorization.
According to Gaertner and Dovidio , a common goal causes perceptions to be reclassified into changed perceptions, thus leading to greater collaboration. It may be expected that if healthcare professionals become aware of a common goal, there will be room to recognize the overlap with the other group of healthcare professionals. This type of development is also known as decategorization. If a certain situation is perceived as a form of decategorization, the emotional group connections become less important, so that there will be room for individuals to recognize shared identities. In turn, this will lead individuals to have more extensive contacts with other individuals, as a result of which prejudices will decrease and positive attitudes towards people in a different group can be developed . This brings us to our third hypothesis:
The positive relation between common goal and collaboration is mediated by the partial effect of decategorization.
Brown and Wade  and Molleman, Broekhuis, Stoffels and Jaspers  conclude that if one wishes to stimulate collaboration, both groups must be able to retain their old identity. It is possible for the two groups to collaborate, but the researchers believe it is important that both groups continue to operate separately and that both fulfil a complementary role within the framework of their common goal. Such a structure, with interdependence and individual space for each group, will ultimately reduce prejudice and tension on either side [21, 37, 38]. This perceived commonality can then lead to greater collaboration [22, 32, 39].
A thorough understanding of interdependence and common endeavour has a psychological effect on interaction, interrelationships and collaboration: it leads to recognition, stimulation and interaction . In the case of mutual differentiation as a social categorization perception, there is appreciation of the knowledge and expertise on the part of the other professional group. According to Gaertner and Dovidio , ‘there is a win-win situation which produces positive feelings and stereotyping towards the other group, while the individual’s own group can define its own profile’. In the case of recategorization and mutual differentiation, it is important in both cases that the original identity is not abandoned when collaboration takes place. Both groups will then be able to retain some autonomy within a common whole and they will not stray too far into each other’s territory .
Where the common goal (in casu One Health) is perceived as collaboration by mutual differentiation, a special focus lies on the importance of each of the groups with respect to their different qualities and expertise. Hewstone and Brown  state that collaboration should be focused on complementary knowledge and expertise. Collaboration will be triggered by paying attention to each other’s knowledge and expertise, as a function of the clarity of a common goal. This brings us to our fourth hypothesis:
The positive relation between common goal and collaboration is mediated by the partial effect of mutual differentiation.
Ives, Torrey and Gordon  argue that having a clear common interest leads to situations in which an exchange of knowledge can take place. In addition to what follows from the Common Ingroup Identity Model, it can be assumed that knowledge sharing leads to collaboration because individuals can use each other’s expertise [42,43,44]. Knowledge sharing between teams and groups improves performance and effectiveness [45,46,47]. Added value can be achieved by having professionals from different backgrounds learning and working together, thanks to the possibilities offered in terms of exchanges, the integration of knowledge and innovation. Advantages are particularly associated with the sharing of implicit knowledge and new insights . Collaboration is promoted by knowledge transfer through informal or small-scale processes and lateral, social contacts [48, 49]. Kramer and Brewer  showed that individuals were particularly inclined to share knowledge with others within their own group, but also that they can be more reticent with more distant contacts. In that case, and especially in the case of One Health, it is important to address perceived distance; when others are perceived as less distant, they will have fewer reservations to collaborate within the framework of One Health. Finally, Holmes  demonstrates a positive connection between (continuing) knowledge sharing and mutual ties, trust within a group and collaboration [22, 32, 51]. This brings us to our fifth hypothesis:
The positive relation between common goal and collaboration is mediated by the partial effect of knowledge sharing.