Science has long considered focus and ability to concentrate as qualities primarily influenced by genetics; basically we're hardwired with varying capacities. But more recent research seems to imply many more factors are involved.
This would explain why, for most people, levels of concentration and focus seem to ebb and flow. Sometimes you can feel sharp as a tack, able to recall facts from long ago, and confident in your ability to focus on a specific task for hours—and other times you feel like you're in a brain fog, feel dazed, or "space out" important details.
The abilities to pay attention, stay focused, remember facts, and even simply sit still for an extended period of time are all affected by diverse systems in the body. Level of focus and concentration can be influenced by:
Diet—Certain foods can heighten or lessen neurological function, especially if an individual has a food sensitivity.
Immune System Health—A balanced immune response and a balanced neurological system go hand-in-hand. Stress in either system can cause stress in the other.
Probiotic Balance—Maintaining healthy gut flora balance can have a direct effect on mental clarity due to the flora's ability to affect pH and biochemistry throughout the body.*
Heavy Metals and Other Environmental Chemicals—The body has a natural ability to detoxify itself. If this inherent detoxification system is compromised or overly stressed, it can affect mental clarity and focus.*
Learned Behaviors—It's the old "nature versus nurture" debate. In the end the answer is: both. Many people learn (or don't learn) lifestyle habits that enhance focus and concentration during formative childhood years.
Age—Memory and focus can be affected by age. However, old and young alike can enhance and maintain mental acuity by supporting the neurological system with good nutrition, supporting the body's ability to detoxify itself, promoting immune system health, and keeping the mind sharp through memory games and continued life-long learning activities.*
Stress—Being under stress for an extended period of time can certainly affect focus. But there are lifestyle choices that can promote a balanced stress response and support healthier focus even in the face of daily stressors.