<p>This definitely varies from culture to culture, tradition and even race. For example in the biblical times first born children are the symbol of a parent's true authenticity. They're entitled to landed properties, special privileges as well as special treatments as well. </p><p>So also let's take a look at the Igbo culture, of the eastern Nigeria in West Africa Their first born children are basically the child that takes responsibilities of replacing their parent if they're absent or maybe dead expecially if they're sons. As a result of this, the parents takes time to focus their attention more on them giving them special privileges as well as placing into their hands matters of urgent attention as well. This definitely shows first born children really gets attention more from their parents more than other sibling. </p><p>However the instances I've sighted had been based on culture and tradition.</p><p>However in a natural phenomenon it may be, it may also not, however in instances where parent have shown more attention to their first born children, it may be because it took them time to have a second and maybe a third  child so they're used to treating their first born specially because that's how it's always been when he/she was the only  child. I definitely think it depends on the type of parents  as well but the percentage are definitely high in most families giving their first born more attention</p>