Sex, gender, and health (differential morbidity and mortality)

Trivers-Willard hypothesis

Under poor conditions, mothers will have fewer male offspring, better conditions will have more male offspring, increasing the fitness of the mother because in poor conditions males will have few offspring but in good conditions that can have many more

Alternative to the Trivers-Willard hypothesis

Males grow more rapidly which makes them more likely to die in bad conditions, not enough food to support quick growth

SDIM

Sex differences in mortality

Sex at birth ratio (SRB)

The number of males born for every 100 females born in the same period of time

Weak male hypothesis

Idea that males were more biologically vulnerable, almost pathologised maleness

What are the sex ratios/sex differences

Number of males : number of females

Equal is 1, excess females <1.0, excess males >1.0

When did sex-based differential infant mortality being to be noticed?

In the 17th to 18th centuries

Found higher male mortality in infancy and early childhood and more males born, countering female weakness idea

Initially believed to be because of urban development, accidents, or injury

Later, weak male hypothesis

How can known differential mortality be used to measure discrimination against female offspring?

106 (or 102-106) males to every 100 females, so a number of males > 106 indicates sex selection or infanticide

Like estimating how many girls were aborted in South Korean, very high proportion of males

But this varies on country, some have more males or more females and the ratio uses European and American birth rates as the default

Historical, economic, and epidemiological context matters

What is Wells’ extended model to investigate differential mortality and what does it suggest?

Extended from just gestation to the whole early childhood period (age of weaning finished, around 3 or 4 years old although not binary)

Would expect that by that point the sex ratio evened out as males died

Malnutrition and infection signal poor conditions biologically (both work together and influence each other)

Both males and females vulnerable to the process, but males are more vulnerable

Suggested that males were less buffered and that environmental stress will always impact males more early in life as they are less able to recover

What are other biological explanations for the excess in early male mortality common across species?

Chromosomal explanation (XX confers greater genetic capacity in the case there is a defect on one X, there is a backup)

Sex-based differences in response to infection due to immunomodulatory functions of sex steroid hormones

Testosterone has a negative impact vs oestrogen is positive

Supposedly higher Th1 response in females

What are critiques of the biological explanations for excess male mortality?

In the 1980s, Stinson argued there was only weak support that males were less buffered

Sex differences in responses to stress may vary depending on the stressor

Humans live in complex cultural environments, may apply to some but not others, social patterns are important

Disease transmission dynamics and behaviour patterns based on sex

If age can impact mortality for a disease, but social conditions mean males get it more when younger, could look like males are more susceptible

Interactions between culture and the environment

What are proposed reasons for changes in sex-based mortality differences over time?

Rather than being a constant, the expression of biological differences is sensitive to the context of the place and time, and is historically contingent

Excess male mortality may be a product of the western demographic and epidemiological transition, as the male survival disadvantage was uncovered (maybe Omran’s theory should be expanded to include a sex difference in mortality transition?)

Cullen et al. (2015), transition in SDIM as the female survival advantage for non-communicable diseases has emerged with the decrease in infectious disease-related deaths

There is an inverse relationship between the infant mortality rate and excess male infant mortality

What were the hypothesised explanations and conclusions regarding the high infant mortality and skewed sex ratio (129 : 100) in the Agta foragers?

Hypothesis 1 = skewed sex ratio was due to systematic female neglect and infanticide

Hypothesis 2 = a higher sex ratio at birth has evolved as an adaptive response to high extrinsic male mortality in this population

Study showed that the sex ratio was due to a biological response to male-biased juvenile mortality

What were the hypothesised reasons for why males had a higher risk of severe disease and a 1.7x higher risk of death from COVID-19 than females after 30?

Oestrogen was protective but androgen wasn’t

Many immune-related genes are on the X chromosome

Females may also be less susceptible to extreme immune responses

But also gender roles and behaviour

These sex differences also depend on the disease, as the way the virus binds to the cell (which is how they sex difference arises) is specific to viruses, so sex differences depend on what the main cause of death is

According to Johansson (1991), why is a historical perspective essential when looking at demography/differential mortality?

Because the relationship between biology, welfare, and longevity is subject to differential social, cultural, and historical contexts, which can all influence each other

Biology is inconsistent and alone, it can’t explain patterns

According to Johansson (1991), why is context an important determinant?

The relationships between biology, sex-specific behaviour, and relative welfare are very complex and vary based on time and place

Context helps to check the observed patterns are real (confounding factors) and to explain patterns and why they change

Usually multiple factors influence each other, contributing to excess or differential mortality

According to Johansson (1991), what factors may influence each other and contribute to excess or differential mortality?

Different disease environments, same behavioural and cultural patterns will have different biological outcomes

Local biologies, we are shaped by our environment

Immune system is adaptive

People’s choices influencing longevity/mortality are constrained by their context (job, diet, etc)

According to Beltrán-Sánchez et al. (2015), what were the causes of higher adult male mortality?

Adult male mortality was higher due to increased rates of heart disease and smoking-related deaths to a lesser degree

According to Beltrán-Sánchez et al. (2015), what factors contributed to higher rates of heart disease and smoking-related deaths among adult males?

Biological, social, and environmental context

Epidemiological transition

Males were more likely to smoke in most places

Differences in diet and lifestyle, men had a higher-fat diet with more meat

Some biological factors, making men more susceptible to changes in lifestyle, like fat distribution and arterial function

What does this graph show?

The Trivers-Willard hypothesis

What does this graph show?

Development of the pattern of excess male mortality in early life in relation to different environmental stresses

The cause of malnutrition is given below the curve, and the associated cause of mortality above it

But approximately four years old the sex difference in mortality disappears

What does this graph show?

The worsening male disadvantage during the major historical decline in infant mortality