Research about military offspring ratio seems to have focused on various environmental and occupational exposures, including:

- radiation

- boron

- g-forces

- polychlorinated biphenyls (PCBs)

- dioxins

- pesticides

- lead, and other metals.

(1961) Snyder Rg. - The sex ratio of offspring of pilots of high performance military aircraft.

"A statistically significant (.01>p>.001) preponderance of female offspring born when fathers flew fighter-type aircraft prior to conception was found. Of 94 children conceived and born under these conditions 59 were females giving a sex ratio males per 100 females of 59.3 as compared with the expected ratio of 105.37 obtained from registered births in the U.S. total population in 1950. It was also found that when fathers flew multiengined types of aircraft prior to conception they fathered 15 females and 25 males a sex ratio of 166.67. When fathers did not fly at all the results were 35 females and 53 males a sex ratio of 151.42. The sex ratio by parity was closer to expected. Normally the sex ratio shows a continuous drop at each succeeding pregnancy. Here when the fathers flew transport types of aircraft or did not fly at all prior to conception the same drop was apparent. However a significant reversal occurred when fathers flew fighters prior to conception of each child: the ratio successively increased with each succeeding birth."

Terrell et al. (2011) said:
Snyder was the only researcher who used detailed military flight logs to calculate the number of flight hours for each pilot, and the only author to focus on flights during the critical window of spermatogenesis.

- (2009) Å. Irgens, L. Irgens. Male proportion in offspring of military air pilots in Norway

"The male proportion was 36.9%, the male proportion in the reference population was 51.4% with a relative risk 0.72 (95% CI 0.57–0.92). "

But there is also research countering this:

- (2019) Kaitlyn Huegel Mayer, Trimble L.B. Spitzer. Offspring gender rates in active duty fighter pilots and flight officers.

reports offspring ratios similar to general population.

Yet, there seems to be an association with more working hours:

- (2009) Rebecca Baczuk, Anthony Biascan, Erik T. Grossgold, Ari Isaacson, J. Spencer. Sex ratio shift in offspring of male fixed-wing naval aviation officers.

"Although the ratio of male offspring from all participants in our study, aircraft groups, and aircraft supergroups was not statistically significantly different from the general population, this study provides statistical support that there is a sex ratio shift among offspring of male aviation officers when they fly more hours during the 11th month before birth. Furthermore, for officers who fl y carrier-based aircraft, that shift is even more dramatic as they approach and exceed 50 hours per month. "

You might think about G-forces but there's also research about submariners:

- (2019) Kathleen E Kramer, S. Raiciulescu, C. Olsen, K. Hickey, M. Ottolini. Altered Sex Ratios in Offspring of U.S. Submariners Urban Legend or Fact - Do Submariners Have More Daughters?

"The submariners surveyed reported greater numbers of daughters than the general population, especially when on sea duty or in regular contact with submarines during shore duty. Further study should be done to confirm results and explore possible etiologies of differences in sex ratio."

We should also note (2011) Terrell states:

"There is some evidence that the endocrine disruptors,
dioxin and PCBs, are associated with altered secondary
sex ratio. Dioxin exposure is associated with a lower
male:female sex ratio in offspring of exposed men, while
PCB exposure is associated with a higher male:female sex
ratio in offspring of exposed men."