I could publish in a near future observations supporting a theory on birth order and phenotypes in individuals of populations stating that children born of females of some populations are displaying decreasing mental aptitudes from firstborns to lastborns and an increasing pathogen resistance from firstborns to the lastborns. According to the theory, the genome of individuals of some populations is using the resources to engender firstborns with highest mental aptitudes possible in an environment with minimal resources to engender pathogen resistance, leading firstborns to die more than lastborns observed, of pathogen infections presumably, sometimes not displaying a minimal resistance to pathogens with too high mental aptitudes from the use of resources of the genome, the nextborns of females engendering firstborns dying could display a higher pathogen resistance with less mental aptitudes and survive to display reproductive behaviours, but for the firstborns who are surviving with minimal pathogen resistance, they are displaying superior mental aptitudes enabling them to engender numerous married children acquiring resources necessary for survival and reproduction displaying such superior mental aptitudes and displaying reproductive behaviours typically associated with these superior mental aptitudes, formation of couples with parental investment toward offspring engendered, in males and in females in some human populations. The laterborns could display inferiour mental aptitudes but a superior resistance to pathogens and display more promiscuous sexual behaviour, marrying less and engendering offspring marrying less, pathogen resistance being necessary for survival and reproduction of individuals of populations, these could reproduce in a lower frequency in human populations. It could explain dozens of observations realized on effect of birth order on survival, mental aptitudes, reproduction and lifespan, my actual observations suggesting that firstborns are living longer than lastborns, coherent with observations on individuals displaying superior mental aptitudes marrying more to reproduce and living longer.