Kevin Leman has wrote three books on the topic of birth order, The Birth Order Book, The New Birth Order Book, and The Birth Order Connection: Finding And Keeping The Love Of Your Life; each book has been widely accepted and the concept of birth order has had mass appeal to do its logical and relevant explanations on human behaviour (Leman NP). .
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Birth order can vary in intensity, largely depending on the severity of the roles played within the developing family; birth order is defined by coping strategies, so the more coping a person has to do in relation to family issues, the more they are affected by birth order (Isaacson NP). As well, there are exceptions to birth order as someone's order of birth can differ from their birth order personality; there is a five year rule between the first and second born children, in that if the second child is born five years after the first child, the birth order trend starts over and both children are then considered to have first born personalities (Stein NP). However, if a third and forth born child is born as much as fourteen years after the child before them, they maintain their consecutive birth order personality (Isaacson NP). There is another birth order phenomenon known as the "ghost child", where a child is born after to the death of the child born before him; this child can take on traits similar to the deceased child before him, complicating the birth order personality of not only him, but the subsequent siblings behind him (Stein NP). .
There is much criticism regarding the birth order theory (Jana 1). Religion based criticism argues that the birth order theory is flawed, and that because it makes no reference to God it simply holds no methodical reasoning (Bobgan NP). It is also felt be skeptics of the theory, that research into birth order has been illegitimate and inconclusive, and therefore, the theory is flawed (Beardsley NP).