The headmasters of the schools were approached individually and were given verbal information based on a schematic diagram of the anatomy of a child in the shape of a puppet. In the diagram the basics of the relation between the pituitary secretion of growth hormone, the liver's synthesis of growth factors, and effects of growth factors on bones were shown (fig 1​1).). Furthermore, the written information, which consisted of a single sheet of paper with 24 lines of text (305 words), (table 1​1),), and the questionnaire were presented to the headmasters.

After this initial approach, separate meetings with the teachers at each school were arranged, which, again, were followed by meetings with the parents of each class over a period of two months. At the meetings the study was presented in accordance with the information given to the headmasters. Teachers and parents were asked to come up with as many questions as possible to better elucidate all aspects of the study. Verbal approval from the parents was obtained to approach their children in class. Furthermore, consent forms were handed out to the parents. However, parents, as well as teachers, were asked not to talk to the children about the study before we had been in contact with them.

Verbal information was given to the children in their classrooms. In all age groups we presented the diagram of a puppet to explain the rationale of the study, and the equipment used for blood and urine sampling was shown to each child so that he or she could become acquainted with it. For the six to ten year old group we created an interactive play using the puppet and ourselves as players to illustrate urine sampling at home, venipuncture at school, weight and height measurements, and a doctor's clinical examination. Furthermore, within a few days of our information sessions a story about a boy who underwent venipuncture for assessment of a growth problem was read to the children by one of their teachers.⁹ In the children 11 to 14 years of age the puppet was used to illustrate urine and blood sampling, weight and height measurements, and a clinical examination; the puppet was not used when giving verbal information to the 15 to 16 year olds. At completion of the verbal information sessions the written information and consent forms were handed out to the children. The children were asked to read the information themselves before asking their parents for help, if needed. Furthermore, they were asked to make up their own mind about giving assent or dissent, if possible, before presenting the information and consent form to their parents. Written consent was given by the parents to the study and/or to participation in the questionnaire.

The questionnaires were handed out to the children in their classes within a week after the children who had assented to the study had completed this study. Members of our research group had talked to the teachers about the questionnaire, so the teachers were fully informed about the meaning of the questions. The questions were read out loud to the children in class by a member of our research group, and the meaning of the questions were explained to them. All children were offered the opportunity to answer the questionnaire in class with access to assistance from a teacher. However, children six to ten years of age were asked to complete the questionnaire at home with their parents, and all children were urged to ask parents for information regarding any question if in doubt.

A few studies have evaluated parent's motives for volunteering their children for therapeutic research; however, children's motives for participating in therapeutic or non‐therapeutic research have not been assessed. Parents may be motivated by a desire to help others and to contribute to medical science and/or by a desire to learn more about their child's illness, and thereby help to make their child better.^10,11,12 In line with the parental motives, the children in the present study volunteered for the non‐therapeutic research project for two significant reasons. They wanted to be of help to children with illnesses and they wanted to gain experience by participating in a study. In a majority of the children the altruistic and educational motives were supported by teachers, parents, and classmates. Remarkably, however, 13–20% of six to eleven year olds and 33–53% of twelve to sixteen year olds had not discussed their decision to assent with their parents. The findings in the oldest age group are in accordance with the results of a questionnaire in which adolescents were asked about their view as to the need for parental consent for adolescents to take part in studies involving blood or urine testing. A total of 42% of the adolescents felt that parental consent was not needed for these procedures.¹³ It was therefore, perhaps, more surprising that so many of the young children in the present study reported that they did not discuss their participation with their parents. We believe, however, that this was a consequence of us informing parents separately and specifically asking them to allow their children to decide about participation on their own if they felt they were able to do so. It must be remembered also that we encouraged the children to make up their own minds about giving assent or dissent before presenting the information and consent form to their parents, which probably helped to explain the fact that assent to the study did not depend on whether or not the children had discussed their participation with parents or classmates.

Though 19–46% of the children had worried about blood sampling, urine collection, or the doctor's examination, only the latter came out as a statistically significant difference between assenters and dissenters. Worries about the doctor's examination were associated with a higher dissent rate. In spite of the worries about these procedures, however, a great majority of children who were worried chose to assent. This suggests that there may be inherent differences between assenters' and dissenters' ability to cope with worries about invasive procedures. To some extent, not being able to cope with worries about having a blood sample taken was probably related to lack of experience of this procedure, since having had a blood sample taken recently was associated with a statistically significantly increase in assent rate.

Studies using objective methods for assessment of parent's and children's consent or assent have found an apparent discrepancy between parent's subjective perception of adequate information and objective assessments of understanding in parents and children.^14,15,16 In an interview study in 102 seven to seventeen year old children and adolescents who participated in anaesthetic or surgery research, the subject's perceived understanding of the core elements of consent was significantly greater than their objectively measured understanding, which varied from 30–90%.¹⁵ In the present study we were interested in assessing whether the perceived understanding of written and verbal information in children who were invited to participate in non‐therapeutic research influenced assent and dissent rates. We agree with the perception that whether or not a child is able to achieve complete understanding of informed consent may be different from whether he or she has the capacity to meaningfully assent or dissent to a study.¹⁷ Therefore, we used the subjective method of asking the children to judge themselves whether they felt they could understand the information given. It was evident that the perception of understanding and the feeling of grasping the information given were associated with increased assent rate. From the age of nine approximately 95% of the children felt they were able to read some or all of the written information, and in the six to eight year old group more than 85% felt that they were able to understand it when they were assisted by a parent. Furthermore, approximately half of the six year olds and more than two thirds in the seven to sixteen year old group felt they understood the verbal information completely; very few felt they did not understand it at all. The findings indicate that when focused one‐page written information is used in combination with age differentiated verbal information, and given in the setting of a school, very few children are left without understanding, and in fact the great majority of them are able to meaningfully assent or dissent to non‐therapeutic research. The low percentage of children who felt they did not understand any of the verbal information could probably have been reduced by giving the children verbal information individually, in addition to the verbal information they were given in class.