Because parents are expected to invest whatever is necessary to ensure the survival of their offspring, it is generally thought that parents will allocate the maximum amount of resources available, possibly to their own detriment and that of other potential offspring. In contrast, close sibling relationships in nonindustrialized cultures are often obligatory, with strong cultural norms prompting cooperation and close proximity between siblings. Cultural differences[ edit ] The content and context of sibling relationships varies between cultures. However, even the most conscientious parents can expect to see sibling rivalry in play to a degree. Children fight more in families where there is no understanding that fighting is not an acceptable way to resolve conflicts, and no alternative ways of handling such conflicts. Parent-offspring conflict theory[ edit ] Formulated by Robert Trivers , parent-offspring theory is important for understanding sibling dynamics and parental decision-making. This view has been largely discredited by modern research. According to Reinisch , studying early sexual behavior generally, over half of all six- and seven-year-old boys have engaged in sex play with other boys, and more than a third of them with girls, while more than a third of six- and seven-year-old girls have engaged in such play with both other girls and with boys. Second, one must determine if the behavior is an isolated incident or part of an enduring pattern: Sibling marriage and incest[ edit ] See also: Goggin and William C. In , a year-old man of Saxony, Germany, who had been imprisoned for three years for fathering four children with his sister appealed unsuccessfully to the European Court of Human Rights. Older siblings report more or less the same level of conflict and rivalry throughout their childhood. According to De Jong, if one or more of these is present, the behavior is abusive, whereas if none is present, the behavior must be considered normal sexual experimentation.