I am one of the few people among my friends who is an only child.  
SO thankful I am an only child with no siblings
I wonder what percentage of people are only children
Not sure but the term applies to people who have never had any siblings at all
I am aware.  I wonder if that means step brothers and sisters too
Only child
An only child is a person with no siblings, either biological or adopted. 
Children who have half-siblings or step-siblings, either living at the same house or a different house - especially those who were born considerably later - may have a similar family environment to only children, as may children who have much younger siblings from both of the same parents (generally ten or more years).
Overview.
Throughout history, only children were relatively uncommon. From around the middle of the
who can possibly save someone's life and I am able to do it at relatively little cost to myself but fail to do so, I have no way of justifying my behaviour to others. If I am one among millions who can save the life of a Bengali orphan by giving to charity, then I only have a limited obligation to that child compatible with others having a similar obligation. That is, I need not reduce myself to the level of marginal utility to help that child: all I need to do is