It is argued that greater involvement of the public in research leads to better research outcomes through improvements to data quality, data collection, and, in time, dissemination [1–6]. This may be particularly true for children and young people because of the likely greater difference between their experiences and those of adult researchers. Issues relevant to young people may be overlooked by adult researchers [7], and data collection tools which are disliked or poorly understood by participants are more likely to be returned incomplete or incorrect. As co-researchers, young people can elicit better data from other young people or facilitate better access to respondents [8]. In dissemination, young people may advise on the accessibility of publications to other young people and in public events can have a greater impact on audiences than adult researchers [8, 9]. Considering ethical practice, researchers must consider when and how to consent children, and consultation with them will improve this process [10, 11]. The small literature that documents changes resulting from greater involvement of the public and participants [2, 12, 13] suggests research improvements, although more evidence is needed.

As well as the potential benefits to research, greater involvement of children and young people has a rights-based dimension. The United Nations Convention on the Rights of the Child includes the right for children to express their views freely in all matters affecting them (Article 12) and for those views to be given due weight in accordance with the age and maturity of the child [14]. Taken further, children’s involvement might be seen as emancipatory; sharing power between the researched (who are a frequently powerless group) and the researcher [9]. There may also be opportunities for children and young people themselves to benefit from research participation; they may gain skills and knowledge through their involvement that will benefit them in later life.

Involving the public in research is also a priority for politicians and funding bodies in health [3, 15]. Methods for involving adult research participants and service users are well developed in some fields [16–19]. Methods for involving children and families are less present. Although good practice exists including in child-led research [20–22], practice in this field is often restricted to service users and qualitative research. There is no intrinsic reason why quantitative research should not also engage in co-production.

Within longitudinal studies, such as birth cohorts, a relationship between researcher and researched must be sustained for many years. Retaining members in the study over the long term is crucial for the study to succeed, but attrition is often a problem [23]. For those convinced of the value of participation, this would seem like the natural home for building a more participatory relationship. Researchers benefit by increased engagement, completion, and (potentially) retention in the study. Study members benefit by having an opportunity to contribute to the research in which they are a part. Birth cohorts (where recruitment is instigated prior to or soon after birth and participants are followed for several years) are currently accorded a high priority in public health research [24]. This interest is evidenced by, for example, the major investment by the ESRC/MRC in the UK Birth Cohort Study and associated large scale facility project [25], and the US National Children’s Study [26]. As well as the necessity of a strong relationship with cohort members, these study types have another advantage for those who advocate participation in the potential created by ongoing data collection for research to respond to the changing world experienced by research participants. Important environmental contexts for health (e.g. trends in use of social media, multi-channel screen time, types and availability of illicit drugs) will likely be better understood in the first instance by those experiencing these changes than by those researching them.

While arguments for participant involvement are well rehearsed, the extent of participant involvement in research practice is often unknown. The study reported here aims to audit and describe the extent and types of consultation, engagement and participation currently used in one type of longitudinal study (birth cohorts) in Europe. We were particularly interested in methods to increase engagement of young cohort members, including but not restricted to opportunities to contribute to research planning. We restricted our sample to Europe in order to make the task manageable and to increase the likelihood we could obtain unpublished details through our existing networks.

While the very smallest cohort were excluded from this group, 200 mother-child pairs is considered a small sample size for cohorts [27] and some smaller cohorts were included where they formed part of multi-cohort studies. Although our primary interest was in recent cohorts (i.e. 1990s onwards), no cut off date was considered appropriate a priori so no studies were excluded on this basis.

Three methods were used to collect data about the engagement practices of these cohorts: 1) information held on study websites and on the ENRIECO and birthcohorts.net websites; 2) further details included in published study documentation; and 3) direct contact with all cohorts. An excel spreadsheet was created to record information on the following topics:

All cohorts were approached at least twice (between October 2011 and March 2012) and asked to provide further information by email or to take part in a telephone interview. Interviews were audio recorded (but not transcribed), and information provided was entered directly into the Excel spreadsheet by the interviewer (DA).

Data were collected regarding response rates and attrition for each cohort (with the intention to compare rates between cohorts) but are not reported in detail here. Study attrition was seldom available, since many cohorts were ongoing and therefore participants could re-enter the study in the future, and reporting of response rates differed between studies [23]. We recorded whole sample response rates to the most recent wave of data collection where this was available, but this information was often missing. The different cohort contexts (size, current age, and geographic spread) together with missing data meant cross cohorts comparisons of retention and attrition was not possible or appropriate.

A total of eighty-four individual cohorts were identified. This represents 70 studies, since some cohorts were linked as a single study (for example the INMA cohorts consists of 7 cohorts from different regions of Spain[28]). We did not find any cohorts which we excluded on the basis of size alone. Sixty-one cohorts had project websites that could be accessed and which we used to obtain additional information about study conduct. Representatives of 32 cohorts (38% of those contacted) responded to emailed requests for additional information, 15 provided additional information via email and we interviewed representatives of 17 cohorts over the phone.

The recent growth in interest and investment in this study design is apparent in the numbers of studies identified. Ten cohorts were established prior to 1990, 23 in the period 1990-1999, 46 between 2000 and 2010 and 4 new cohorts since 2011. Most (n=73, 87%) of the studies collect data related to physical health only, but 11 also include measures of mental health, development, and social context more broadly. For most of these studies, response rates present a considerable problem, particularly over the longer term. Among the 23 cohorts that recruited during the period 1990-1999, response rates more than 10 years after enrolment varied between 13% and 84% for individual data rounds. Among ongoing studies, members could re-enter the study even after missing several data sweeps so it is difficult to say how many have to been lost to follow up.

Cohorts also actively stay in touch with their members; sending out newsletters with summaries of findings. We know of 20 studies which send information newsletters, or information about upcoming events or data collection waves, 3 which send cards (usually for birthdays), and 4 which give participants health advice. Five cohorts also hold events for families in their cohorts (such as parties) on an annual or occasional basis.

We collected data on the location, focus, size, and start date of the cohorts (shown in Table 2). Newer cohorts were more likely to have websites, but since most cohorts maintain websites differences by decade are small. Only 1 of the smallest cohorts (≤500 members) maintained a website, but aside from this size and efforts to engage or consult with cohort members did not seem to be related. The age of cohorts did not seem to be related to how likely they were to engage with participants, with both some of the oldest and some of the youngest cohorts providing opportunities for feedback to the study teams.

Finally we asked study groups who agreed to be interviewed what made it difficult, or had discouraged them from greater use of consultation with their young participants. There were 5 barriers identified: concerns about impact on quality of research, ethical constraints, resource limitations, lack of importance, and difficulties experienced to date. Some study groups were concerned that further contact with participants would compromise research quality because of increased respondent burden and a risk of increased attrition or because of Hawthorne effects whereby the responses of their study group would be influenced by greater contact with the study team. In addition a number of ethical considerations were raised: some studies were bound by the original consents obtained which would not allow additional contacts with young participants; there was also a concern that any activity that might bring together study members would breach anonymity; and finally there were felt to be ethical constraints in working with the children independent of their parents. Resource limitations were a significant barrier; several study groups felt that they didn’t have sufficient resources for this work particularly where cohort members did not live near each other and where children of different ages were represented in a cohort. Related to lack of resource, was a belief that this kind of work might be interesting, but it either was not important or was not something they had considered, so it was not among their priorities. Where rates of retention were good, researchers felt there was no need for further engagement and thus the costs of doing so were not justified. Finally, some researchers felt that participatory work was difficult to get right, and past experience made them pessimistic that they would be able to achieve this.

This study shows that most European birth cohort studies undertake some activities to promote wider engagement in their ongoing work, but this is most often restricted to project websites. Very few actively and regularly consult with their members. A few others have facilities for participant feedback in response to data collection. This suggests that participant consultation is low among the priorities for this group as a whole, albeit that some are making concerted attempts to do so.

This study identified varied practice in consultation with parent and child cohort members; most send information to participants and reward participation, a few have feedback mechanisms for cohort members, very rarely there are opportunities for members to contribute to research planning through consultation. Barriers to increased consultation were identified as fears for research quality, ethical concerns, lack of resources, poor past experiences, and a belief this wasn’t necessary. These barriers have been overcome in some cohorts and in other fields, and these provide examples of how more consultation could be undertaken within the scope of funding and resources. The large increase in cohorts initiated in the last 10 years means that there are a large number of children who are soon to be old enough for consultation to take place.