How We Do It

The evolution and future of human reproduction

Robert D. Martin Ph.D.

Do Men Have the Balls for Promiscuity?

The myth that humans are biologically adapted for promiscuous mating with sperm competition is widely accepted but directly conflicts with data on testis size and sperm dimensions.

Monogamy Anchored in Our Genes?

Humans are often seen as biologically primed for monogamy. Absence of adaptations for sperm competition indicates single-male mating, but more likely mild polygyny than monogamy.

Sex on a Cycle?

Studies of women’s sexual motivation often assume an influence of hormonal changes across the menstrual cycle. But the expected sexuality peak around ovulation is commonly absent.

Beauty and the Beard

Darwin suggested that men’s beards evolved through sexual selection exerted by female mate choice. But recent studies indicate that beards may primarily signal social dominance.

Breastfeeding No Option? Women Need Remedies, Not Bullying

Courtney Jung’s book "Lactivism" rightly condemns bullying of mothers who do not breastfeed but misrepresents abundant evidence demonstrating substantial benefits of breastfeeding.

Fake Sex Hormones: Chemicals Damage Health and Reproduction

DDT and DES were just the beginning of a series of environmental chemicals that act as fake hormones, seriously disrupting our health and reproduction.

Masturbation: Self-Abuse or Biological Necessity?

Masturbation has slowly lost its evil reputation from previous centuries. Human studies and primate research both indicate that it brings benefits rather than health risks.

Bigfoot Parents Have Small Brains

Birds and mammals mostly show intensive parenting, linked to their “warm-blooded” nature and quite large brains. Incubator birds show no care of their chicks and have tiny brains.

A Sperm’s Obstacle Course to the Egg

If too many sperms surround the egg, catastrophic fertilization by more than one may occur. A woman’s reproductive tract is adapted to tightly constrain sperm numbers near the egg.

Why Too Many Sperms Spoil the Egg

Men can produce too many sperms as well as too few. Excessive sperm numbers around an egg can result in multiple fertilization, abnormal chromosome numbers, and miscarriage.

Twins and More: Too Much of a Good Thing?

Multiple pregnancies bring more bundles of joy, but premature birth is more common. Delayed motherhood and fertility treatments are boosting the frequency of multiple births.

Twins With Two Daddies

Paternity suits triggered by strikingly different fraternal twins have revealed bizarre cases with two different fathers. Split paternity is the visible tip of a sizeable iceberg.

The Rising Tide of Caesarean Births

C-sections have been multiplying out of control worldwide. Evidence for side-effects has mounted in tandem, but in the USA at least it seems that the tide may have turned.

Going, Going, Gone? Human Sperm Counts Are Plunging

Accumulated evidence confirms that human sperm counts have fallen markedly in industrialized countries since the 1930s. Male fertility and reproductive health are threatened.

The Benefits of Breastfeeding...for Mothers

Breast-feeding benefits mothers, not just babies. It speeds recovery of the womb after birth. Later in life, risks of heart disease and breast and ovarian cancer are reduced.

St. Valentine’s Day and the Biology of Romantic Kissing

For Westerners, kisses and Valentine’s Day are inseparable. But new cross-cultural evidence challenges evolutionary inferences regarding possible biological functions of kissing.

Dining on Leftovers: Do Women Lose by Not Eating Afterbirth?

Unlike all other primates and most other mammals, human mothers do not usually devour the placenta after birth. Are they missing out on important benefits because of this?

Bonking Without Bones: An Evolutionary Enigma

New studies clarify the evolution of a "penis bone" in most primates—except humans.

Pandora’s Pillbox: The Age-Old Quest for Longer Lives

Prolonging human lifespan is a long-cherished goal. Medical science has boosted average life expectancy, but can it increase the maximum? Not according to evolutionary biology.

Coping With Hormonal Ups and Downs

Major hormonal changes across the menstrual cycle may influence a woman’s mood, well-being and performance. But apart from PMS side-effects are milder than often thought.

Intimately Connected

Research sheds new light on biological origins of women’s sexuality.

A Downside for Trophy Wives: Sexually Diminished Husbands

Increasing reproductive problems with age are by no means confined to women. Clear evidence of similar drawbacks for men has been widely ignored. Trophy wives pay a high price!

Unmanned: An Unnatural History of Human Castration

Deliberate castration, long used to punish criminals and prisoners of war, has also yielded harem guards and has been inflicted on young boys to stop their voices from breaking.

Honey, I Squished the Kids’ Heads!

Around the world, people have tightly bound their babies’ heads to produce distinctive shapes. Extreme cases in South America have spawned wild stories of intervention by aliens.

Summertime Sex and Springtime Babies

Human populations have regular year-round birth patterns. Births typically peak in spring in the north but 6 months later in the south, hinting at control by an internal clock.

Beating the Biological Clock with Eggs in the Freezer

A widespread tendency to delay pregnancy clashes with reduced fertility and increased risks later in life. Will deep-freezing eggs of young women for later use solve the problems?

What Dogs Can Tell Us About Sex and Conception

Sperm survival in the womb for 10 days or more rules out the standard model of human conception.

A Biological Function for Oral Sex?

High blood pressure affects 10% of pregnancies in which a mother’s immune system over-reacts to the fetus. Long-term sexual priming by a specific male partner reduces the risk.

The Ripley Effect: Alien Intruders in the Womb

A fetus in its mother’s womb must somehow evade detection by her immune system. In an "Alien"-like scenario, genes that permit this have been captured from once harmful viruses.

Sleeping With Baby: Good or Bad?

A crucial decision parents must make after birth is whether to sleep with the infant. Some claim that sudden infant death is more likely. But bed-sharing promotes breast-feeding.

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