When a woman sends a warm and inviting smile to a male stranger she may see a confused look on his face, or at least a strained return smile. That is because he must instantly deal with two messages: one from his body and one from his conscious mind. And the messages are most likely opposite.
His conscious mind has learned to be polite and observe the rules taught in his family and culture about dealing with strangers and especially women. At the same time his body starts revving up for action as the old brain instantly kick-starts it with a flood of hormones.
Lab research reveals that many chemical and physical changes take place in a man's body without him noticing when a woman stranger smiles at him. The same response happens even when he looks at a picture of a woman smiling!
It appears that healthy young men's bodies are designed to go into action when seeing a woman smiling at him. Our smile remains from our ancient ancestors as the most obvious body language cue for indicating we are safe to approach. Additionally, men are DNA programmed from their ancient ancestors to not miss a chance for sex. That is what worked best for men to keep the species from going extinct in those hard times. Another indication of men's readiness for sex is the fact that young healthy men make 200 million to 300 million sperm a day!