It probably depends who you ask. Mom and Dad say no, but you know better. In my family we had our theories growing up--we knew who needed to ask for things we all wanted. Most parents I know (including my own) insist on not showing favoritism, even if they feel just a tinge of it for a child. Honestly, (as the baby of my family) I don't understand the cultures, ancient or modern, that prize firstborn sons over other children. It's so foreign to my worldview.
Biblical culture often appears to encourage the opposite. Genesis has a complicated mixture of both.
Underneath many of the stories in Genesis lies a tension between cultural expectation and reality, especially in regard to firstborn sons. Whether it's Ishmael then Isaac, Esau then Jacob, or Joseph as one of the last children of Israel, we see Genesis operating within, and often subverting, a culture that prized firstborn children with extra honor, love, and inheritance. The narrative of Cain and Abel (and later Seth), fits into this tension, but often receives less attention.
The expected child of promise is born to Adam and Eve with her adding, "I have gotten a man with the help of YHWH" (Gen 4:1). This line adds even more to the expectation. YHWH is directly involved in this promised son's birth. In the narrative there is an unmistakable optimism about this son named Cain.
And then everything goes wrong.
By the end of the paragraph we see Cain in uncontrolled anger because his sacrifice was rejected. (In my opinion, this is because he does not give his firstfruits of the harvest as Abel gave the firstborn of his flock. In Genesis, firstborns are supposed to belong to God, whether of the field, the flock, or even the family.) The anger presents a problem and a choice: "Sin is crouching at the door. Its desire is for you, but you must rule over it." Almost quoting the curse language of Gen 3:15-16, the author reminds us again of God's plans to reverse the curse through the woman's seed (pay attention later to the curse language in Gen 5:29).
Cain's break towards bad spirals further downward, away from YHWH. Premeditated murder. No remorse. Outright rebellion is his ultimate end as he is sentenced to wandering, yet he still builds his son a city (a wicked theme I'll pick up later in Genesis 11).
In Cain we see a picture of hope subverted into violent rebellion. In him we see a man meant for more, but seeking it the wrong way. As I wrote in the last post, Wisdom and Life go hand in hand, coming from the fear of YHWH. But at every corner are the voices of deception and foolishness, ending in death. Proverbs 1:10-11 put it this way: "My son, if sinners entice you, do not consent. If they say, 'Come with us, let us lie in wait for blood; let us ambush the innocent without reason."
Like father like son, as the Proverbs often go... if only the father had told this son.
The Wisdom instruction plays out again in narrative form. The rest of Genesis 4 is a genealogy showing the contagiousness of folly as we overview Cain's seed on down to Lamech. Wickedness and pomposity perpetuated through Cain's line for generations, ending in polygamy, murder, and proud protests to YHWH. Neither Cain nor his seed will be the victor over the serpent. But if not Cain, then who else? Cain rejects God's will for his life, and Abel is dead.
That certainly didn't play out like we expected or hoped.
But even in this we see something remarkable and unique about YHWH. YHWH works through the unexpected, in providence he powers through human failure. Over and over in Genesis and in history, God uses the unexpected to show his power and to fulfill his purposes.
The way this shows through here hardly interests many modern readers. A genealogical disaster in Genesis 4:17-24 is followed by yet another genealogy in chapter 5. This double dose of genealogy may not excite you, but it is crucial to moving along the story. It moves us quickly into the future, and it plays out the consequences of the lifestyles of the great characters, generations into the future. In fact, "these are the generations of" is the phrase that gives structure to the entire book (11 times--2:4; 5:1*; 6:9; 10:1; 11:10; 11:27; 25:12; 25:19; 36:1; 36:9; 37:2). In the family lines of Cain and Seth (Adam), we get a picture of which families God uses and why. Sometimes it happens in the most unexpected ways.
In this case, God won't work through the firstborn (Cain), and it is certainly won't be Abel (whose name is the Hebrew word for "vanity" or "vapor" as in Ecclesiastes--here for a moment and then done too soon). Enter Seth--born as his mother says, "God has appointed me another offspring." Seth is the replacement for Abel, and through his descendants we don't see violence, cities, and defiance. Instead, we meet the people who begin worshiping YHWH (Gen 4:26). Genesis 5 is the genealogy of the people whom God will use to fulfill his promise to reverse the curse (cf. Gen 5:29). They are flawed people, yes, but a people who will repeatedly call out to God. A people through him God will be victorious, however unexpectedly.
(Thank God for this genealogy!)