Why might ADHD be missed in children and adults?
There are many reasons that an adult with ADHD might not have been diagnosed as a child. The growing awareness and knowledge of ADHD is relatively new. Many of today's adults did not benefit from this awareness as children.
Most cultures have various gender biases and expectations. In western culture, we commonly expect boys to be rambunctious and girls to be demure dreamers. In fact, girls are more often diagnosed with ADHD inattentive type in contrast to the Hyperactive/impulsive type that is more commonly assigned to boys. Gender and other cultural biases may mask certain diagnostic symptoms when used to normalize or excuse a culturally accepted behavior, "Oh you know, that's just how Bobby is. Boys will be boys." Gender biases likewise make it socially acceptable for girls to be quiet and demure. These cultural, gender expectations have also allowed girls with ADHD to go unnoticed. Their inattentive symptoms may be similarly written off as, "Oh, she's such a daydreamer."
Other socio-cultural factors may obscure an accurate ADHD diagnosis. Previous generations married, and started families, at much younger ages. The focus of these younger, earlier generation parents was quite necessarily to meet the practical demands of their own adulthood and families; e.g., so-called 'bread and butter' needs. Certain knowledge about childrearing was unknown and each family did the best they could, given their own familial background. The knowledge and information about what is normal childhood behavior was not part of public information, nor could a parent simply consult Google®. All these factors converged so that parents today are more knowledgeable and aware of childhood health concerns. However, these past socio-cultural dynamics allowed many symptomatic children to go undiagnosed.
Still, you might reasonably ask, "Yes, but, how could someone not realize there is something wrong them? It seems like it would be hard not to notice." That may be true. However, some adults with ADHD (particularly highly intelligent ones) may develop very effective compensatory habits that serve to manage (and conceal) their ADHD symptoms. These exceptionally well-developed coping strategies may mask the identification of the disorder. For example, adults may develop highly-structured routines just to avoid forgetting important tasks. We might think to ourselves, "Wow, they sure are addicted to that smartphone!" However, the truth is that smartphone serves as a critically-needed, organizational tool.
These compensatory coping mechanisms must be considered during the evaluation as it may impact treatment decisions. Effective coping behaviors may allow someone to forgo medication. In such a case, treatment might focus on expanding these coping strategies for maximum effectiveness. For others, medication may be a great benefit to help them to master effective coping skills.